diff -rup vim74/src/option.c vim74-new/src/option.c
--- vim74/src/option.c	2013-07-17 12:39:13.000000000 -0700
+++ vim74-new/src/option.c	2016-11-28 16:59:31.763889780 -0800
@@ -5653,6 +5653,21 @@ set_string_option(opt_idx, value, opt_fl
 }
 
 /*
+ * Return TRUE if "val" is a valid 'filetype' name.
+ * Also used for 'syntax' and 'keymap'.
+ */
+    static int
+valid_filetype(char_u *val)
+{
+    char_u *s;
+
+    for (s = val; *s != NUL; ++s)
+	if (!ASCII_ISALNUM(*s) && vim_strchr((char_u *)".-_", *s) == NULL)
+	    return FALSE;
+    return TRUE;
+}
+
+/*
  * Handle string options that need some action to perform when changed.
  * Returns NULL for success, or an error message for an error.
  */
@@ -6043,8 +6058,11 @@ did_set_string_option(opt_idx, varp, new
 #ifdef FEAT_KEYMAP
     else if (varp == &curbuf->b_p_keymap)
     {
-	/* load or unload key mapping tables */
-	errmsg = keymap_init();
+	if (!valid_filetype(*varp))
+	    errmsg = e_invarg;
+	else
+	    /* load or unload key mapping tables */
+	    errmsg = keymap_init();
 
 	if (errmsg == NULL)
 	{
@@ -6990,6 +7008,22 @@ did_set_string_option(opt_idx, varp, new
     }
 #endif
 
+#ifdef FEAT_AUTOCMD
+    else if (gvarp == &p_ft)
+    {
+	if (!valid_filetype(*varp))
+	    errmsg = e_invarg;
+    }
+#endif
+
+#ifdef FEAT_SYN_HL
+    else if (gvarp == &p_syn)
+    {
+	if (!valid_filetype(*varp))
+	    errmsg = e_invarg;
+    }
+#endif
+
     /* Options that are a list of flags. */
     else
     {