diff --git a/jerry-core/parser/js/js-parser-statm.c b/jerry-core/parser/js/js-parser-statm.c
index 00d8bdce0..e82c0e6e6 100644
--- a/jerry-core/parser/js/js-parser-statm.c
+++ b/jerry-core/parser/js/js-parser-statm.c
@@ -710,9 +710,9 @@ parser_parse_function_statement (parser_context_t *context_p) /**< context */
                 && context_p->token.lit_location.type == LEXER_IDENT_LITERAL);
 
 #if ENABLED (JERRY_ES2015)
-  if (context_p->next_scanner_info_p->source_p == context_p->source_p)
+  if (context_p->next_scanner_info_p->source_p == context_p->source_p
+      && context_p->next_scanner_info_p->type == SCANNER_TYPE_ERR_REDECLARED)
   {
-    JERRY_ASSERT (context_p->next_scanner_info_p->type == SCANNER_TYPE_ERR_REDECLARED);
     parser_raise_error (context_p, PARSER_ERR_VARIABLE_REDECLARED);
   }
 #endif /* ENABLED (JERRY_ES2015) */
@@ -730,8 +730,6 @@ parser_parse_function_statement (parser_context_t *context_p) /**< context */
     status_flags |= PARSER_HAS_NON_STRICT_ARG;
   }
 
-  JERRY_ASSERT (context_p->next_scanner_info_p->type == SCANNER_TYPE_FUNCTION);
-
 #if ENABLED (JERRY_ES2015)
   if (is_generator_function)
   {
diff --git a/tests/jerry/regression-test-issue-3821.js b/tests/jerry/regression-test-issue-3821.js
new file mode 100644
index 000000000..6fca983f0
--- /dev/null
+++ b/tests/jerry/regression-test-issue-3821.js
@@ -0,0 +1,35 @@
+// 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 check_syntax_error (script)
+{
+  try
+  {
+    eval (script);
+    assert (false);
+  }
+  catch (e)
+  {
+    assert (e instanceof SyntaxError);
+  }
+}
+
+check_syntax_error ('function')
+check_syntax_error ('function a')
+check_syntax_error ('function a"')
+check_syntax_error ('function a"b')
+check_syntax_error ('function a("b')
+check_syntax_error ('function a(b, "c')
+check_syntax_error ('function a(b, c"d')
+check_syntax_error ('function a(b, c)"d')