diff --git a/qutebrowser/keyinput/keyutils.py b/qutebrowser/keyinput/keyutils.py
index d89f9aa5c..fd00cf5b5 100644
--- a/qutebrowser/keyinput/keyutils.py
+++ b/qutebrowser/keyinput/keyutils.py
@@ -38,7 +38,18 @@ _MODIFIER_MAP = {
 }
 
 
+def _assert_plain_key(key):
+    """Make sure this is a key without KeyboardModifiers mixed in."""
+    assert not key & Qt.KeyboardModifierMask, hex(key)
+
+
+def _assert_plain_modifier(key):
+    """Make sure this is a modifier without a key mixed in."""
+    assert not key & ~Qt.KeyboardModifierMask, hex(key)
+
+
 def is_printable(key):
+    _assert_plain_key(key)
     return key <= 0xff and key not in [Qt.Key_Space, 0x0]
 
 
@@ -48,6 +59,7 @@ def is_modifier_key(key):
     This only considers keys which are part of Qt::KeyboardModifiers, i.e.
     which would interrupt a key chain like "yY" when handled.
     """
+    _assert_plain_key(key)
     return key in _MODIFIER_MAP
 
 
@@ -73,6 +85,7 @@ def _key_to_string(key):
     Return:
         A name of the key as a string.
     """
+    _assert_plain_key(key)
     special_names_str = {
         # Some keys handled in a weird way by QKeySequence::toString.
         # See https://bugreports.qt.io/browse/QTBUG-40030
@@ -155,6 +168,7 @@ def _modifiers_to_string(modifiers):
     Handles Qt.GroupSwitchModifier because Qt doesn't handle that as a
     modifier.
     """
+    _assert_plain_modifier(modifiers)
     if modifiers & Qt.GroupSwitchModifier:
         modifiers &= ~Qt.GroupSwitchModifier
         result = 'AltGr+'
@@ -446,6 +460,9 @@ class KeySequence:
         key = ev.key()
         modifiers = ev.modifiers()
 
+        _assert_plain_key(key)
+        _assert_plain_modifier(modifiers)
+
         if key == 0x0:
             raise KeyParseError(None, "Got nil key!")
 
diff --git a/tests/unit/keyinput/test_keyutils.py b/tests/unit/keyinput/test_keyutils.py
index 097b72b71..0ec8814f1 100644
--- a/tests/unit/keyinput/test_keyutils.py
+++ b/tests/unit/keyinput/test_keyutils.py
@@ -444,7 +444,6 @@ def test_key_info_to_event():
     (Qt.Key_Return, False),
     (Qt.Key_Enter, False),
     (Qt.Key_Space, False),
-    (Qt.Key_X | Qt.ControlModifier, False),  # Wrong usage
     (0x0, False),  # Used by Qt for unknown keys
 
     (Qt.Key_ydiaeresis, True),
@@ -461,3 +460,16 @@ def test_is_printable(key, printable):
 ])
 def test_is_modifier_key(key, ismodifier):
     assert keyutils.is_modifier_key(key) == ismodifier
+
+
+@pytest.mark.parametrize('code', [
+    lambda a: keyutils._assert_plain_key(a),
+    lambda a: keyutils._assert_plain_modifier(a),
+    lambda a: keyutils.is_printable(a),
+    lambda a: keyutils.is_modifier_key(a),
+    lambda a: keyutils._key_to_string(a),
+    lambda a: keyutils._modifiers_to_string(a),
+])
+def test_non_plain(code):
+    with pytest.raises(AssertionError):
+        code(Qt.Key_X | Qt.ControlModifier)