diff --git a/jerry-core/ecma/operations/ecma-function-object.c b/jerry-core/ecma/operations/ecma-function-object.c
index 53778ace5a..bf3ee1ccb1 100644
--- a/jerry-core/ecma/operations/ecma-function-object.c
+++ b/jerry-core/ecma/operations/ecma-function-object.c
@@ -641,7 +641,8 @@ ecma_op_function_has_instance (ecma_object_t *func_obj_p, /**< Function object *
   }
 
   JERRY_ASSERT (ecma_get_object_type (func_obj_p) == ECMA_OBJECT_TYPE_FUNCTION
-                || ecma_get_object_type (func_obj_p) == ECMA_OBJECT_TYPE_EXTERNAL_FUNCTION);
+                || ecma_get_object_type (func_obj_p) == ECMA_OBJECT_TYPE_EXTERNAL_FUNCTION
+                || ECMA_OBJECT_IS_PROXY (func_obj_p));
 
   ecma_object_t *v_obj_p = ecma_get_object_from_value (value);
 
diff --git a/tests/jerry/es2015/regression-test-issue-3814.js b/tests/jerry/es2015/regression-test-issue-3814.js
new file mode 100644
index 0000000000..08024236e8
--- /dev/null
+++ b/tests/jerry/es2015/regression-test-issue-3814.js
@@ -0,0 +1,17 @@
+// Copyright JS Foundation and other contributors, http://js.foundation
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+function f () {}
+
+assert((new f) instanceof new Proxy(f, {}))