diff --git a/builtin/ls-files.c b/builtin/ls-files.c
index 1592290815c8b93701fa9d175d76f7a7ed85f7d2..18105ec7ea0cea4bd30c3d4aa8a2fb29423f678c 100644
--- a/builtin/ls-files.c
+++ b/builtin/ls-files.c
@@ -369,11 +369,14 @@ static void show_files(struct dir_struct *dir)
 /*
  * Prune the index to only contain stuff starting with "prefix"
  */
-static void prune_cache(const char *prefix)
+static void prune_cache(const char *prefix, size_t prefixlen)
 {
-	int pos = cache_name_pos(prefix, max_prefix_len);
+	int pos;
 	unsigned int first, last;
 
+	if (!prefix)
+		return;
+	pos = cache_name_pos(prefix, prefixlen);
 	if (pos < 0)
 		pos = -pos-1;
 	memmove(active_cache, active_cache + pos,
@@ -384,7 +387,7 @@ static void prune_cache(const char *prefix)
 	while (last > first) {
 		int next = (last + first) >> 1;
 		const struct cache_entry *ce = active_cache[next];
-		if (!strncmp(ce->name, prefix, max_prefix_len)) {
+		if (!strncmp(ce->name, prefix, prefixlen)) {
 			first = next+1;
 			continue;
 		}
@@ -641,8 +644,7 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 	      show_killed || show_modified || show_resolve_undo))
 		show_cached = 1;
 
-	if (max_prefix)
-		prune_cache(max_prefix);
+	prune_cache(max_prefix, max_prefix_len);
 	if (with_tree) {
 		/*
 		 * Basic sanity check; show-stages and show-unmerged