diff --git a/vlib/builtin/int.v b/vlib/builtin/int.v
index d5e56d245c..ce1191fb5e 100644
--- a/vlib/builtin/int.v
+++ b/vlib/builtin/int.v
@@ -150,7 +150,7 @@ pub fn (n i64) hex() string {
 		19
 	}
 	hex := malloc(len)
-	count := int(C.sprintf(*char(hex), '0x%llx', n))
+	count := int(C.sprintf(*char(hex), '0x%' C.PRIx64, n))
 	return tos(hex, count)
 }