diff --git a/mitmproxy/addons/clientplayback.py b/mitmproxy/addons/clientplayback.py
index 7f2b53ac6..e69cd27ab 100644
--- a/mitmproxy/addons/clientplayback.py
+++ b/mitmproxy/addons/clientplayback.py
@@ -21,7 +21,7 @@ class ClientPlayback:
     def configure(self, options, updated):
         if "client_replay" in updated:
             if options.client_replay:
-                ctx.log.info(options.client_replay)
+                ctx.log.info("Client Replay: {}".format(options.client_replay))
                 try:
                     flows = io.read_flows_from_paths(options.client_replay)
                 except exceptions.FlowReadException as e:
diff --git a/mitmproxy/options.py b/mitmproxy/options.py
index 26b7030e7..035471895 100644
--- a/mitmproxy/options.py
+++ b/mitmproxy/options.py
@@ -23,13 +23,14 @@ DEFAULT_CLIENT_CIPHERS = "ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA
 class Options(optmanager.OptManager):
     def __init__(
             self,
+            *,  # all args are keyword-only.
             # TODO: rename to onboarding_app_*
             app: bool = True,
             app_host: str = APP_HOST,
             app_port: int = APP_PORT,
             anticache: bool = False,
             anticomp: bool = False,
-            client_replay: Optional[str] = None,
+            client_replay: Sequence[str] = (),
             replay_kill_extra: bool = False,
             keepserving: bool = True,
             no_server: bool = False,
@@ -41,12 +42,12 @@ class Options(optmanager.OptManager):
             replacements: Sequence[Tuple[str, str, str]] = (),
             server_replay_use_headers: Sequence[str] = (),
             setheaders: Sequence[Tuple[str, str, str]] = (),
-            server_replay: Sequence[str] = None,
+            server_replay: Sequence[str] = (),
             stickycookie: Optional[str] = None,
             stickyauth: Optional[str] = None,
-            stream_large_bodies: Optional[str] = None,
+            stream_large_bodies: Optional[int] = None,
             verbosity: int = 2,
-            outfile: Tuple[str, str] = None,
+            outfile: Optional[Tuple[str, str]] = None,
             server_replay_ignore_content: bool = False,
             server_replay_ignore_params: Sequence[str] = (),
             server_replay_ignore_payload_params: Sequence[str] = (),
@@ -71,13 +72,13 @@ class Options(optmanager.OptManager):
             rawtcp: bool = False,
             websockets: bool = False,
             spoof_source_address: bool = False,
-            upstream_server: str = "",
-            upstream_auth: str = "",
+            upstream_server: Optional[str] = None,
+            upstream_auth: Optional[str] = None,
             ssl_version_client: str = "secure",
             ssl_version_server: str = "secure",
             ssl_insecure: bool = False,
-            ssl_verify_upstream_trusted_cadir: str = None,
-            ssl_verify_upstream_trusted_ca: str = None,
+            ssl_verify_upstream_trusted_cadir: Optional[str] = None,
+            ssl_verify_upstream_trusted_ca: Optional[str] = None,
             tcp_hosts: Sequence[str] = ()
     ):
         # We could replace all assignments with clever metaprogramming,
diff --git a/mitmproxy/optmanager.py b/mitmproxy/optmanager.py
index 6683e41dd..20492f829 100644
--- a/mitmproxy/optmanager.py
+++ b/mitmproxy/optmanager.py
@@ -3,6 +3,7 @@ import blinker
 import pprint
 
 from mitmproxy import exceptions
+from mitmproxy.utils import typecheck
 
 """
     The base implementation for Options.
@@ -58,10 +59,19 @@ class OptManager:
 
     def __setattr__(self, attr, value):
         if not self._initialized:
+            self._typecheck(attr, value)
             self._opts[attr] = value
             return
         self.update(**{attr: value})
 
+    def _typecheck(self, attr, value):
+        expected_type = typecheck.get_arg_type_from_constructor_annotation(
+            type(self), attr
+        )
+        if expected_type is None:
+            return  # no type info :(
+        typecheck.check_type(attr, value, expected_type)
+
     def keys(self):
         return set(self._opts.keys())
 
@@ -70,9 +80,10 @@ class OptManager:
 
     def update(self, **kwargs):
         updated = set(kwargs.keys())
-        for k in kwargs:
+        for k, v in kwargs.items():
             if k not in self._opts:
                 raise KeyError("No such option: %s" % k)
+            self._typecheck(k, v)
         with self.rollback(updated):
             self._opts.update(kwargs)
             self.changed.send(self, updated=updated)
diff --git a/mitmproxy/tools/cmdline.py b/mitmproxy/tools/cmdline.py
index 2f9ea15c6..55adb7fa0 100644
--- a/mitmproxy/tools/cmdline.py
+++ b/mitmproxy/tools/cmdline.py
@@ -591,7 +591,7 @@ def client_replay(parser):
     group = parser.add_argument_group("Client Replay")
     group.add_argument(
         "-c", "--client-replay",
-        action="append", dest="client_replay", default=None, metavar="PATH",
+        action="append", dest="client_replay", default=[], metavar="PATH",
         help="Replay client requests from a saved file."
     )
 
@@ -600,7 +600,7 @@ def server_replay(parser):
     group = parser.add_argument_group("Server Replay")
     group.add_argument(
         "-S", "--server-replay",
-        action="append", dest="server_replay", default=None, metavar="PATH",
+        action="append", dest="server_replay", default=[], metavar="PATH",
         help="Replay server responses from a saved file."
     )
     group.add_argument(
@@ -610,7 +610,7 @@ def server_replay(parser):
     )
     group.add_argument(
         "--server-replay-use-header",
-        action="append", dest="server_replay_use_headers", type=str,
+        action="append", dest="server_replay_use_headers", type=str, default=[],
         help="Request headers to be considered during replay. "
              "Can be passed multiple times."
     )
@@ -638,7 +638,7 @@ def server_replay(parser):
     )
     payload.add_argument(
         "--replay-ignore-payload-param",
-        action="append", dest="server_replay_ignore_payload_params", type=str,
+        action="append", dest="server_replay_ignore_payload_params", type=str, default=[],
         help="""
             Request's payload parameters (application/x-www-form-urlencoded or multipart/form-data) to
             be ignored while searching for a saved flow to replay.
@@ -648,7 +648,7 @@ def server_replay(parser):
 
     group.add_argument(
         "--replay-ignore-param",
-        action="append", dest="server_replay_ignore_params", type=str,
+        action="append", dest="server_replay_ignore_params", type=str, default=[],
         help="""
             Request's parameters to be ignored while searching for a saved flow
             to replay. Can be passed multiple times.
diff --git a/mitmproxy/tools/console/master.py b/mitmproxy/tools/console/master.py
index 909c83da6..c128e42b5 100644
--- a/mitmproxy/tools/console/master.py
+++ b/mitmproxy/tools/console/master.py
@@ -203,9 +203,10 @@ class ConsoleState(state.State):
 class Options(mitmproxy.options.Options):
     def __init__(
             self,
+            *,  # all args are keyword-only.
             eventlog: bool = False,
             follow: bool = False,
-            intercept: bool = False,
+            intercept: Optional[str] = None,
             filter: Optional[str] = None,
             palette: Optional[str] = None,
             palette_transparent: bool = False,
diff --git a/mitmproxy/tools/dump.py b/mitmproxy/tools/dump.py
index e92482f39..837959bcf 100644
--- a/mitmproxy/tools/dump.py
+++ b/mitmproxy/tools/dump.py
@@ -18,6 +18,7 @@ class DumpError(Exception):
 class Options(options.Options):
     def __init__(
             self,
+            *,  # all args are keyword-only.
             keepserving: bool = False,
             filtstr: Optional[str] = None,
             flow_detail: int = 1,
diff --git a/mitmproxy/tools/web/master.py b/mitmproxy/tools/web/master.py
index 619582f34..75842422c 100644
--- a/mitmproxy/tools/web/master.py
+++ b/mitmproxy/tools/web/master.py
@@ -94,8 +94,9 @@ class WebState(state.State):
 class Options(options.Options):
     def __init__(
             self,
+            *,  # all args are keyword-only.
             intercept: Optional[str] = None,
-            wdebug: bool = bool,
+            wdebug: bool = False,
             wport: int = 8081,
             wiface: str = "127.0.0.1",
             wauthenticator: Optional[authentication.PassMan] = None,
diff --git a/mitmproxy/utils/typecheck.py b/mitmproxy/utils/typecheck.py
new file mode 100644
index 000000000..ce57cff10
--- /dev/null
+++ b/mitmproxy/utils/typecheck.py
@@ -0,0 +1,54 @@
+import typing
+
+
+def check_type(attr_name: str, value: typing.Any, typeinfo: type) -> None:
+    """
+    This function checks if the provided value is an instance of typeinfo
+    and raises a TypeError otherwise.
+
+    The following types from the typing package have specialized support:
+
+    - Union
+    - Tuple
+    - TextIO
+    """
+    # If we realize that we need to extend this list substantially, it may make sense
+    # to use typeguard for this, but right now it's not worth the hassle for 16 lines of code.
+
+    e = TypeError("Expected {} for {}, but got {}.".format(
+        typeinfo,
+        attr_name,
+        type(value)
+    ))
+
+    if isinstance(typeinfo, typing.UnionMeta):
+        for T in typeinfo.__union_params__:
+            try:
+                check_type(attr_name, value, T)
+            except TypeError:
+                pass
+            else:
+                return
+        raise e
+    if isinstance(typeinfo, typing.TupleMeta):
+        check_type(attr_name, value, tuple)
+        if len(typeinfo.__tuple_params__) != len(value):
+            raise e
+        for i, (x, T) in enumerate(zip(value, typeinfo.__tuple_params__)):
+            check_type("{}[{}]".format(attr_name, i), x, T)
+        return
+    if typeinfo == typing.TextIO:
+        if hasattr(value, "read"):
+            return
+
+    if not isinstance(value, typeinfo):
+        raise e
+
+
+def get_arg_type_from_constructor_annotation(cls: type, attr: str) -> typing.Optional[type]:
+    """
+    Returns the first type annotation for attr in the class hierarchy.
+    """
+    for c in cls.mro():
+        if attr in getattr(c.__init__, "__annotations__", ()):
+            return c.__init__.__annotations__[attr]
diff --git a/setup.py b/setup.py
index 1351ba73a..4165b3675 100644
--- a/setup.py
+++ b/setup.py
@@ -67,7 +67,7 @@ setup(
         "cryptography>=1.3, <1.6",
         "cssutils>=1.0.1, <1.1",
         "Flask>=0.10.1, <0.12",
-        "h2>=2.4.1, <3",
+        "h2>=2.4.1, <2.5",
         "html2text>=2016.1.8, <=2016.9.19",
         "hyperframe>=4.0.1, <5",
         "jsbeautifier>=1.6.3, <1.7",
diff --git a/test/mitmproxy/utils/__init__.py b/test/mitmproxy/utils/__init__.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/test/mitmproxy/test_utils_data.py b/test/mitmproxy/utils/test_data.py
similarity index 52%
rename from test/mitmproxy/test_utils_data.py
rename to test/mitmproxy/utils/test_data.py
index c6e4420e8..f40fc8665 100644
--- a/test/mitmproxy/test_utils_data.py
+++ b/test/mitmproxy/utils/test_data.py
@@ -1,7 +1,8 @@
+import pytest
 from mitmproxy.utils import data
-from . import tutils
 
 
 def test_pkg_data():
     assert data.pkg_data.path("tools/console")
-    tutils.raises("does not exist", data.pkg_data.path, "nonexistent")
+    with pytest.raises(ValueError):
+        data.pkg_data.path("nonexistent")
diff --git a/test/mitmproxy/test_utils_debug.py b/test/mitmproxy/utils/test_debug.py
similarity index 100%
rename from test/mitmproxy/test_utils_debug.py
rename to test/mitmproxy/utils/test_debug.py
diff --git a/test/mitmproxy/test_utils_human.py b/test/mitmproxy/utils/test_human.py
similarity index 100%
rename from test/mitmproxy/test_utils_human.py
rename to test/mitmproxy/utils/test_human.py
diff --git a/test/mitmproxy/test_utils_strutils.py b/test/mitmproxy/utils/test_strutils.py
similarity index 100%
rename from test/mitmproxy/test_utils_strutils.py
rename to test/mitmproxy/utils/test_strutils.py
diff --git a/test/mitmproxy/utils/test_typecheck.py b/test/mitmproxy/utils/test_typecheck.py
new file mode 100644
index 000000000..85684df94
--- /dev/null
+++ b/test/mitmproxy/utils/test_typecheck.py
@@ -0,0 +1,48 @@
+import typing
+
+import pytest
+from mitmproxy.utils import typecheck
+
+
+class TBase:
+    def __init__(self, bar: int):
+        pass
+
+
+class T(TBase):
+    def __init__(self, foo: str):
+        super(T, self).__init__(42)
+
+
+def test_get_arg_type_from_constructor_annotation():
+    assert typecheck.get_arg_type_from_constructor_annotation(T, "foo") == str
+    assert typecheck.get_arg_type_from_constructor_annotation(T, "bar") == int
+    assert not typecheck.get_arg_type_from_constructor_annotation(T, "baz")
+
+
+def test_check_type():
+    typecheck.check_type("foo", 42, int)
+    with pytest.raises(TypeError):
+        typecheck.check_type("foo", 42, str)
+    with pytest.raises(TypeError):
+        typecheck.check_type("foo", None, str)
+
+
+def test_check_union():
+    typecheck.check_type("foo", 42, typing.Union[int, str])
+    typecheck.check_type("foo", "42", typing.Union[int, str])
+    with pytest.raises(TypeError):
+        typecheck.check_type("foo", [], typing.Union[int, str])
+
+
+def test_check_tuple():
+    with pytest.raises(TypeError):
+        typecheck.check_type("foo", None, typing.Tuple[int, str])
+    with pytest.raises(TypeError):
+        typecheck.check_type("foo", (), typing.Tuple[int, str])
+    with pytest.raises(TypeError):
+        typecheck.check_type("foo", (42, 42), typing.Tuple[int, str])
+    with pytest.raises(TypeError):
+        typecheck.check_type("foo", ("42", 42), typing.Tuple[int, str])
+
+    typecheck.check_type("foo", (42, "42"), typing.Tuple[int, str])
diff --git a/test/mitmproxy/test_utils_version_check.py b/test/mitmproxy/utils/test_version_check.py
similarity index 100%
rename from test/mitmproxy/test_utils_version_check.py
rename to test/mitmproxy/utils/test_version_check.py