diff --git a/src/help.c b/src/help.c
index ee6ff18..67e4fb2 100644
--- a/src/help.c
+++ b/src/help.c
@@ -422,8 +422,7 @@ find_help_tags(
 		    || (vim_strchr((char_u *)"%_z@", arg[1]) != NULL
 							   && arg[2] != NUL)))
 	{
-	    STRCPY(d, "/\\\\");
-	    STRCPY(d + 3, arg + 1);
+	    vim_snprintf((char *)d, IOSIZE, "/\\\\%s", arg + 1);
 	    // Check for "/\\_$", should be "/\\_\$"
 	    if (d[3] == '_' && d[4] == '$')
 		STRCPY(d + 4, "\\$");
diff --git a/src/testdir/test_help.vim b/src/testdir/test_help.vim
index ff2bc41..c8ff5b8 100644
--- a/src/testdir/test_help.vim
+++ b/src/testdir/test_help.vim
@@ -123,5 +123,13 @@ func Test_helptag_cmd_readonly()
   call delete('Xdir', 'rf')
 endfunc
 
+func Test_help_long_argument()
+  try
+    exe 'help \%' .. repeat('0', 1021)
+  catch
+    call assert_match("E149:", v:exception)
+  endtry
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab