diff --git a/ext/json/json.c b/ext/json/json.c
index b57129b1e07..6f4571241d1 100644
--- a/ext/json/json.c
+++ b/ext/json/json.c
@@ -439,6 +439,7 @@ PHP_FUNCTION(json_decode)
 
 	if (str_type == IS_UNICODE) {
 		utf16 = str.u;
+		utf16_len = str_len;
 	} else {
 		utf16 = (unsigned short *) emalloc((str_len+1) * sizeof(unsigned short));