diff --git a/types/extra-types.d.ts b/types/extra-types.d.ts
index c4d38f5fb5..8326a0df2e 100644
--- a/types/extra-types.d.ts
+++ b/types/extra-types.d.ts
@@ -17,7 +17,7 @@ interface PolymerElementPropertiesMeta {
 }
 
 type PolymerElementProperties = {
-  [key: string]: PolymerElementPropertiesMeta
+  [key: string]: PolymerElementPropertiesMeta | Function;
 };
 
 // TODO Document these properties.