diff --git a/src/optionstr.c b/src/optionstr.c
index 8458f2a..b7cdcc4 100644
--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -2582,12 +2582,13 @@
     if (*matches == NULL)
 	return FAIL;
 
-    size_t pattern_len = STRLEN(xp->xp_pattern);
+    int pattern_len = xp->xp_pattern_len;
 
     for (i = 0; i < num_hl_modes; i++)
     {
 	// Don't allow duplicates as these are unique flags
-	if (vim_strchr(xp->xp_pattern + 1, p_hl_mode_values[i]) != NULL)
+	char_u *dup = vim_strchr(xp->xp_pattern + 1, p_hl_mode_values[i]);
+	if (dup != NULL && (int)(dup - xp->xp_pattern) < pattern_len)
 	    continue;
 
 	// ':' only works by itself, not with other flags.