diff --git a/qemu/include/qapi/qmp/qstring.h b/qemu/include/qapi/qmp/qstring.h
index 0a563f87..f143f755 100644
--- a/qemu/include/qapi/qmp/qstring.h
+++ b/qemu/include/qapi/qmp/qstring.h
@@ -25,7 +25,7 @@ struct QString {
 
 QString *qstring_new(void);
 QString *qstring_from_str(const char *str);
-QString *qstring_from_substr(const char *str, int start, int end);
+QString *qstring_from_substr(const char *str, size_t start, size_t end);
 size_t qstring_get_length(const QString *qstring);
 const char *qstring_get_str(const QString *qstring);
 const char *qstring_get_try_str(const QString *qstring);
diff --git a/qemu/qobject/qstring.c b/qemu/qobject/qstring.c
index b4bee272..fcf4ba2b 100644
--- a/qemu/qobject/qstring.c
+++ b/qemu/qobject/qstring.c
@@ -37,7 +37,7 @@ size_t qstring_get_length(const QString *qstring)
  *
  * Return string reference
  */
-QString *qstring_from_substr(const char *str, int start, int end)
+QString *qstring_from_substr(const char *str, size_t start, size_t end)
 {
     QString *qstring;