--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -2878,17 +2878,16 @@ nsRuleNode::SetDefaultOnRoot(const nsSty
     default:
       /*
        * unhandled case: nsStyleStructID_Length.
        * last item of nsStyleStructID, to know its length.
        */
       MOZ_ASSERT(false, "unexpected SID");
       return nullptr;
   }
-  return nullptr;
 }
 
 /**
  * Begin an nsRuleNode::Compute*Data function for an inherited struct.
  *
  * @param type_ The nsStyle* type this function computes.
  * @param data_ Variable (declared here) holding the result of this
  *              function.