diff --git a/src/testdir/check.vim b/src/testdir/check.vim
index d01dd81..22c33c0 100644
--- a/src/testdir/check.vim
+++ b/src/testdir/check.vim
@@ -25,9 +25,12 @@
   endif
 endfunc
 
-" Command to check for the presence of a function.
+" Command to check for the presence of a built-in function.
 command -nargs=1 CheckFunction call CheckFunction(<f-args>)
 func CheckFunction(name)
+  if !exists('?' .. a:name)
+    throw 'Checking for non-existent function ' .. a:name
+  endif
   if !exists('*' .. a:name)
     throw 'Skipped: ' .. a:name .. ' function missing'
   endif