--- a/layout/style/ServoCSSRuleList.cpp
+++ b/layout/style/ServoCSSRuleList.cpp
@@ -270,17 +270,17 @@ ServoCSSRuleList::InsertRule(const nsASt
       StyleSheet* sheet = mStyleSheet->GetMostRecentlyAddedChildSheet();
       MOZ_ASSERT(sheet, "Should have at least one "
                  "child stylesheet after inserting @import rule");
       ServoStyleSheet* servoSheet = sheet->AsServo();
       // This should always be that case, but currently ServoStyleSheet
       // may be reused and the reused stylesheet doesn't refer to the
       // right raw sheet, which is bug 1368381. This should be converted
       // to an assertion after that bug gets fixed.
-      if (servoSheet->RawSheet() == raw) {
+      if (servoSheet->RawSheet() != raw) {
         NS_WARNING("New child sheet should always be prepended to the list");
         return static_cast<ServoStyleSheet*>(nullptr);
       }
       return servoSheet;
     });
   }
   return rv;
 }