diff --git a/builtin-string.go b/builtin-string.go
index 23f1be9..1ac0f6b 100644
--- a/builtin-string.go
+++ b/builtin-string.go
@@ -66,11 +66,11 @@ func subStrFunc(ctx ExprContext, name string, args []any) (result any, err error
 		return nil, ErrWrongParamType(name, ParamSource, TypeString, args[0])
 	}
 
-	if start, err = ToInt(args[1], name+"()"); err != nil {
+	if start, err = ToGoInt(args[1], name+"()"); err != nil {
 		return
 	}
 
-	if count, err = ToInt(args[2], name+"()"); err != nil {
+	if count, err = ToGoInt(args[2], name+"()"); err != nil {
 		return
 	}