diff --git a/ts/smartdata.classes.doc.ts b/ts/smartdata.classes.doc.ts
index 051f428..36107c9 100644
--- a/ts/smartdata.classes.doc.ts
+++ b/ts/smartdata.classes.doc.ts
@@ -41,7 +41,7 @@ export function unI() {
   };
 }
 
-export class SmartDataDbDoc<T, TImplements> {
+export class SmartDataDbDoc<T extends TImplements, TImplements> {
   /**
    * the collection object an Doc belongs to
    */