diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 712aec4..d342e51 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -2280,6 +2280,7 @@
   call assert_equal('a', nr2char(97, 0))
 
   call assert_equal("\x80\xfc\b\xf4\x80\xfeX\x80\xfeX\x80\xfeX", eval('"\<M-' .. nr2char(0x100000) .. '>"'))
+  call assert_equal("\x80\xfc\b\xfd\x80\xfeX\x80\xfeX\x80\xfeX\x80\xfeX\x80\xfeX", eval('"\<M-' .. nr2char(0x40000000) .. '>"'))
 endfunc
 
 " Test for screenattr(), screenchar() and screenchars() functions