diff --git a/qutebrowser/test/test_immutabledict.py b/qutebrowser/test/test_immutabledict.py
new file mode 100644
index 000000000..6aca6d9dd
--- /dev/null
+++ b/qutebrowser/test/test_immutabledict.py
@@ -0,0 +1,44 @@
+# Copyright 2014 Florian Bruhin (The Compiler) <mail@qutebrowser.org>
+#
+# This file is part of qutebrowser.
+#
+# qutebrowser is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# qutebrowser is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with qutebrowser.  If not, see <http://www.gnu.org/licenses/>.
+
+# pylint: disable=missing-docstring
+
+"""Tests for the FakeDict class."""
+
+import unittest
+from unittest import TestCase
+
+from qutebrowser.utils.usertypes import ImmutableDict
+
+
+class ImmutableDictTests(TestCase):
+
+    """Test the ImmutableDict usertype."""
+
+    def setUp(self):
+        self.d = ImmutableDict()
+
+    def test_setitem(self):
+        self.d['foo'] = 'bar'
+        self.assertEqual(self.d['foo'], 'bar')
+        with self.assertRaises(ValueError):
+            self.d['foo'] = 'baz'
+        self.assertEqual(self.d['foo'], 'bar')
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/qutebrowser/utils/usertypes.py b/qutebrowser/utils/usertypes.py
index a67a24e49..94d0113ce 100644
--- a/qutebrowser/utils/usertypes.py
+++ b/qutebrowser/utils/usertypes.py
@@ -182,3 +182,13 @@ class FakeDict:
 
     def __getitem__(self, _key):
         return self._val
+
+
+class ImmutableDict(dict):
+
+    """Dict where items only can be set once."""
+
+    def __setitem__(self, key, value):
+        if key in self:
+            raise ValueError("Key {} already has been set.".format(key))
+        super().__setitem__(key, value)