diff --git a/qutebrowser/config/conftypes.py b/qutebrowser/config/conftypes.py
index eb099ca01..c8edfc5a4 100644
--- a/qutebrowser/config/conftypes.py
+++ b/qutebrowser/config/conftypes.py
@@ -120,6 +120,10 @@ class String(BaseType):
     def transform(self, value):
         return value.lower()
 
+    def validate(self, value):
+        # Nothing to do
+        return
+
 
 class Bool(BaseType):