diff --git a/src/testdir/test_expr.vim b/src/testdir/test_expr.vim
index ed8fa2b..39e719a 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -373,8 +373,10 @@
   call assert_fails('echo printf("%d", [])', 'E745:')
   call assert_fails('echo printf("%d", 1, 2)', 'E767:')
   call assert_fails('echo printf("%*d", 1)', 'E766:')
+  call assert_fails('echo printf("%s")', 'E766:')
   if has('float')
     call assert_fails('echo printf("%d", 1.2)', 'E805:')
+    call assert_fails('echo printf("%f")')
   endif
 endfunc