diff --git a/lib/fields.js b/lib/fields.js
index <HASH>..<HASH> 100644
--- a/lib/fields.js
+++ b/lib/fields.js
@@ -42,7 +42,7 @@ exports.string = function (opt) {
                 if (b.required) {
                     var validator = typeof b.required === 'function' ? b.required : validators.required();
                     validator(form, b, function (v_err) {
-                        b.error = v_err ? v_err.toString() : null;
+                        b.error = v_err ? String(v_err) : null;
                         callback(v_err, b);
                     });
                 } else {
@@ -52,7 +52,7 @@ exports.string = function (opt) {
                 async.forEachSeries(b.validators || [], function (v, callback) {
                     if (!b.error) {
                         v(form, b, function (v_err) {
-                            b.error = v_err ? v_err.toString() : null;
+                            b.error = v_err ? String(v_err) : null;
                             callback(null);
                         });
                     } else {
Use String() instead of the toString prototype method.