diff --git a/src/testdir/test_json.vim b/src/testdir/test_json.vim
index 93597cb..f881ede 100644
--- a/src/testdir/test_json.vim
+++ b/src/testdir/test_json.vim
@@ -196,6 +196,7 @@
   call assert_fails('call json_decode("{[]:42}")', "E491:")
 
   call assert_fails('call json_decode("-")', "E491:")
+  call assert_fails('call json_decode("-1x")', "E491:")
   call assert_fails('call json_decode("infinit")', "E491:")
 
   call assert_fails('call json_decode("\"\\u111Z\"")', 'E491:')