diff --git a/ext/phar/phar.c b/ext/phar/phar.c
index 3cd29f6506f..961be29fd3e 100644
--- a/ext/phar/phar.c
+++ b/ext/phar/phar.c
@@ -1835,7 +1835,7 @@ static int phar_check_str(const char *fname, const char *ext_str, int ext_len, i
 	char test[51];
 	const char *pos;
 
-	if (ext_len >= 50) {
+	if (ext_len < 0 || ext_len >= 50) {
 		return FAILURE;
 	}