diff --git a/src/InterfaceTypeComposer.js b/src/InterfaceTypeComposer.js
index cf0bde23..509236b1 100644
--- a/src/InterfaceTypeComposer.js
+++ b/src/InterfaceTypeComposer.js
@@ -1092,6 +1092,7 @@ export class InterfaceTypeComposer<TSource, TContext> {
     const typeResolversMap = this.getTypeResolvers();
     typeResolversMap.set(type, checkFn);
     this.setTypeResolvers(typeResolversMap);
+    this.schemaComposer.addSchemaMustHaveType(type);
     return this;
   }
 
diff --git a/src/UnionTypeComposer.js b/src/UnionTypeComposer.js
index 75364d6f..577f8726 100644
--- a/src/UnionTypeComposer.js
+++ b/src/UnionTypeComposer.js
@@ -621,6 +621,7 @@ export class UnionTypeComposer<TSource, TContext> {
     const typeResolversMap = this.getTypeResolvers();
     const tc = this._convertObjectType(type);
     typeResolversMap.set(tc, checkFn);
+    this.schemaComposer.addSchemaMustHaveType((tc: any));
     this.setTypeResolvers(typeResolversMap);
     return this;
   }