diff --git a/mitmproxy/command.py b/mitmproxy/command.py
index 8f0755254..c94e8abb2 100644
--- a/mitmproxy/command.py
+++ b/mitmproxy/command.py
@@ -35,9 +35,11 @@ def typename(t: type) -> str:
     """
         Translates a type to an explanatory string.
     """
+    if t == inspect._empty:  # type: ignore
+        raise exceptions.CommandError("missing type annotation")
     to = mitmproxy.types.CommandTypes.get(t, None)
     if not to:
-        raise NotImplementedError(t)
+        raise exceptions.CommandError("unsupported type: %s" % getattr(t, "__name__", t))
     return to.display
 
 
@@ -58,7 +60,12 @@ class Command:
             if i.kind == i.VAR_POSITIONAL:
                 self.has_positional = True
         self.paramtypes = [v.annotation for v in sig.parameters.values()]
-        self.returntype = sig.return_annotation
+        if sig.return_annotation == inspect._empty:  # type: ignore
+            self.returntype = None
+        else:
+            self.returntype = sig.return_annotation
+        # This fails with a CommandException if types are invalid
+        self.signature_help()
 
     def paramnames(self) -> typing.Sequence[str]:
         v = [typename(i) for i in self.paramtypes]
@@ -133,7 +140,12 @@ class CommandManager(mitmproxy.types._CommandBase):
                     pass  # hasattr may raise if o implements __getattr__.
                 else:
                     if is_command:
-                        self.add(o.command_path, o)
+                        try:
+                            self.add(o.command_path, o)
+                        except exceptions.CommandError as e:
+                            self.master.log.warn(
+                                "Could not load command %s: %s" % (o.command_path, e)
+                            )
 
     def add(self, path: str, func: typing.Callable):
         self.commands[path] = Command(self, path, func)
diff --git a/mitmproxy/tools/console/commands.py b/mitmproxy/tools/console/commands.py
index 1183ee9d4..0f35742b3 100644
--- a/mitmproxy/tools/console/commands.py
+++ b/mitmproxy/tools/console/commands.py
@@ -46,12 +46,13 @@ class CommandItem(urwid.WidgetWrap):
 class CommandListWalker(urwid.ListWalker):
     def __init__(self, master):
         self.master = master
-
         self.index = 0
-        self.focusobj = None
-        self.cmds = list(master.commands.commands.values())
+        self.refresh()
+
+    def refresh(self):
+        self.cmds = list(self.master.commands.commands.values())
         self.cmds.sort(key=lambda x: x.signature_help())
-        self.set_focus(0)
+        self.set_focus(self.index)
 
     def get_edit_text(self):
         return self.focus_obj.get_edit_text()
@@ -137,6 +138,9 @@ class Commands(urwid.Pile, layoutwidget.LayoutWidget):
         )
         self.master = master
 
+    def layout_pushed(self, prev):
+        self.widget_list[0].walker.refresh()
+
     def keypress(self, size, key):
         if key == "m_next":
             self.focus_position = (
diff --git a/test/mitmproxy/test_command.py b/test/mitmproxy/test_command.py
index ea1017e7c..7c0dc06d9 100644
--- a/test/mitmproxy/test_command.py
+++ b/test/mitmproxy/test_command.py
@@ -1,4 +1,5 @@
 import typing
+import inspect
 from mitmproxy import command
 from mitmproxy import flow
 from mitmproxy import exceptions
@@ -55,7 +56,35 @@ class TAddon:
         pass
 
 
+class Unsupported:
+    pass
+
+
+class TypeErrAddon:
+    @command.command("noret")
+    def noret(self):
+        pass
+
+    @command.command("invalidret")
+    def invalidret(self) -> Unsupported:
+        pass
+
+    @command.command("invalidarg")
+    def invalidarg(self, u: Unsupported):
+        pass
+
+
 class TestCommand:
+    def test_typecheck(self):
+        with taddons.context(loadcore=False) as tctx:
+            cm = command.CommandManager(tctx.master)
+            a = TypeErrAddon()
+            command.Command(cm, "noret", a.noret)
+            with pytest.raises(exceptions.CommandError):
+                command.Command(cm, "invalidret", a.invalidret)
+            with pytest.raises(exceptions.CommandError):
+                command.Command(cm, "invalidarg", a.invalidarg)
+
     def test_varargs(self):
         with taddons.context() as tctx:
             cm = command.CommandManager(tctx.master)
@@ -275,6 +304,11 @@ def test_typename():
     assert command.typename(mitmproxy.types.Path) == "path"
     assert command.typename(mitmproxy.types.Cmd) == "cmd"
 
+    with pytest.raises(exceptions.CommandError, match="missing type annotation"):
+        command.typename(inspect._empty)
+    with pytest.raises(exceptions.CommandError, match="unsupported type"):
+        command.typename(None)
+
 
 class DummyConsole:
     @command.command("view.resolve")
@@ -326,7 +360,8 @@ class TCmds(TAttr):
         pass
 
 
-def test_collect_commands():
+@pytest.mark.asyncio
+async def test_collect_commands():
     """
         This tests for the error thrown by hasattr()
     """
@@ -336,6 +371,10 @@ def test_collect_commands():
         c.collect_commands(a)
         assert "empty" in c.commands
 
+        a = TypeErrAddon()
+        c.collect_commands(a)
+        await tctx.master.await_log("Could not load")
+
 
 def test_decorator():
     with taddons.context() as tctx: