diff --git a/lib/src/source_linker.dart b/lib/src/source_linker.dart
index 220b518..aad2963 100644
--- a/lib/src/source_linker.dart
+++ b/lib/src/source_linker.dart
@@ -113,6 +113,8 @@
         case '%l%':
           return lineNumber.toString();
           break;
+        default:
+          return null;
       }
     });
   }