diff --git a/cssutils/util.py b/cssutils/util.py
index c21ec90c..ed48cfcc 100644
--- a/cssutils/util.py
+++ b/cssutils/util.py
@@ -817,12 +817,11 @@ def __setitem__(self, prefix, namespaceURI):
 
     def __findrule(self, prefix):
         # returns namespace rule where prefix == key
-        for rule in filter(
-            lambda r: r.type == r.NAMESPACE_RULE,
+        found = filter(
+            lambda r: r.type == r.NAMESPACE_RULE and r.prefix == prefix,
             reversed(self.parentStyleSheet.cssRules),
-        ):
-            if rule.prefix == prefix:
-                return rule
+        )
+        return next(found, None)
 
     @property
     def namespaces(self):