diff --git a/mitmproxy/addons/__init__.py b/mitmproxy/addons/__init__.py
index 165106403..80e3b2cbf 100644
--- a/mitmproxy/addons/__init__.py
+++ b/mitmproxy/addons/__init__.py
@@ -4,7 +4,7 @@ from mitmproxy.addons import check_alpn
 from mitmproxy.addons import check_ca
 from mitmproxy.addons import clientplayback
 from mitmproxy.addons import core_option_validation
-from mitmproxy.addons import disable_h2c_upgrade
+from mitmproxy.addons import disable_h2c
 from mitmproxy.addons import onboarding
 from mitmproxy.addons import proxyauth
 from mitmproxy.addons import replace
@@ -26,7 +26,7 @@ def default_addons():
         check_alpn.CheckALPN(),
         check_ca.CheckCA(),
         clientplayback.ClientPlayback(),
-        disable_h2c_upgrade.DisableH2CleartextUpgrade(),
+        disable_h2c.DisableH2C(),
         onboarding.Onboarding(),
         proxyauth.ProxyAuth(),
         replace.Replace(),
diff --git a/mitmproxy/addons/disable_h2c.py b/mitmproxy/addons/disable_h2c.py
new file mode 100644
index 000000000..b43d52073
--- /dev/null
+++ b/mitmproxy/addons/disable_h2c.py
@@ -0,0 +1,41 @@
+import mitmproxy
+
+
+class DisableH2C:
+
+    """
+    We currently only support HTTP/2 over a TLS connection.
+
+    Some clients try to upgrade a connection from HTTP/1.1 to h2c. We need to
+    remove those headers to avoid protocol errors if one endpoints suddenly
+    starts sending HTTP/2 frames.
+
+    Some clients might use HTTP/2 Prior Knowledge to directly initiate a session
+    by sending the connection preface. We just kill those flows.
+    """
+
+    def configure(self, options, updated):
+        pass
+
+    def process_flow(self, f):
+        if f.request.headers.get('upgrade', '') == 'h2c':
+            mitmproxy.ctx.log.warn("HTTP/2 cleartext connections (h2c upgrade requests) are currently not supported.")
+            del f.request.headers['upgrade']
+            if 'connection' in f.request.headers:
+                del f.request.headers['connection']
+            if 'http2-settings' in f.request.headers:
+                del f.request.headers['http2-settings']
+
+        is_connection_preface = (
+            f.request.method == 'PRI' and
+            f.request.path == '*' and
+            f.request.http_version == 'HTTP/2.0'
+        )
+        if is_connection_preface:
+            f.kill()
+            mitmproxy.ctx.log.warn("Initiating HTTP/2 connections with prior knowledge are currently not supported.")
+
+    # Handlers
+
+    def request(self, f):
+        self.process_flow(f)
diff --git a/mitmproxy/addons/disable_h2c_upgrade.py b/mitmproxy/addons/disable_h2c_upgrade.py
deleted file mode 100644
index f4a36d5f0..000000000
--- a/mitmproxy/addons/disable_h2c_upgrade.py
+++ /dev/null
@@ -1,21 +0,0 @@
-class DisableH2CleartextUpgrade:
-
-    """
-    We currently only support HTTP/2 over a TLS connection. Some clients try
-    to upgrade a connection from HTTP/1.1 to h2c, so we need to remove those
-    headers to avoid protocol errors if one endpoints suddenly starts sending
-    HTTP/2 frames.
-    """
-
-    def process_flow(self, f):
-        if f.request.headers.get('upgrade', '') == 'h2c':
-            del f.request.headers['upgrade']
-            if 'connection' in f.request.headers:
-                del f.request.headers['connection']
-            if 'http2-settings' in f.request.headers:
-                del f.request.headers['http2-settings']
-
-    # Handlers
-
-    def request(self, f):
-        self.process_flow(f)
diff --git a/test/mitmproxy/addons/test_disable_h2c.py b/test/mitmproxy/addons/test_disable_h2c.py
new file mode 100644
index 000000000..d4df8390d
--- /dev/null
+++ b/test/mitmproxy/addons/test_disable_h2c.py
@@ -0,0 +1,39 @@
+import io
+from mitmproxy import http
+from mitmproxy.addons import disable_h2c
+from mitmproxy.net.http import http1
+from mitmproxy.exceptions import Kill
+from mitmproxy.test import tflow
+from mitmproxy.test import taddons
+
+
+class TestDisableH2CleartextUpgrade:
+    def test_upgrade(self):
+        with taddons.context() as tctx:
+            a = disable_h2c.DisableH2C()
+            tctx.configure(a)
+
+            f = tflow.tflow()
+            f.request.headers['upgrade'] = 'h2c'
+            f.request.headers['connection'] = 'foo'
+            f.request.headers['http2-settings'] = 'bar'
+
+            a.request(f)
+            assert 'upgrade' not in f.request.headers
+            assert 'connection' not in f.request.headers
+            assert 'http2-settings' not in f.request.headers
+
+    def test_prior_knowledge(self):
+        with taddons.context() as tctx:
+            a = disable_h2c.DisableH2C()
+            tctx.configure(a)
+
+            b = io.BytesIO(b"PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n")
+            f = tflow.tflow()
+            f.request = http.HTTPRequest.wrap(http1.read_request(b))
+            f.reply.handle()
+            f.intercept()
+
+            a.request(f)
+            assert not f.killable
+            assert f.reply.value == Kill
diff --git a/test/mitmproxy/addons/test_disable_h2c_upgrade.py b/test/mitmproxy/addons/test_disable_h2c_upgrade.py
deleted file mode 100644
index 6cab713d1..000000000
--- a/test/mitmproxy/addons/test_disable_h2c_upgrade.py
+++ /dev/null
@@ -1,17 +0,0 @@
-from mitmproxy.addons import disable_h2c_upgrade
-from mitmproxy.test import tflow
-
-
-class TestTermLog:
-    def test_simple(self):
-        a = disable_h2c_upgrade.DisableH2CleartextUpgrade()
-
-        f = tflow.tflow()
-        f.request.headers['upgrade'] = 'h2c'
-        f.request.headers['connection'] = 'foo'
-        f.request.headers['http2-settings'] = 'bar'
-
-        a.request(f)
-        assert 'upgrade' not in f.request.headers
-        assert 'connection' not in f.request.headers
-        assert 'http2-settings' not in f.request.headers