diff --git a/src/etapi/etapi-interface.ts b/src/etapi/etapi-interface.ts
index b53d983dd..a78b98c2c 100644
--- a/src/etapi/etapi-interface.ts
+++ b/src/etapi/etapi-interface.ts
@@ -1,4 +1,4 @@
-type ValidatorArg = object | undefined | null;
+type ValidatorArg = object | string | undefined | null;
 
 type ValidatorFunc = (obj: ValidatorArg) => (string | undefined);