diff --git a/Source/BuiltInBasicType.swift b/Source/BuiltInBasicType.swift
index 8e44c18..daa848b 100644
--- a/Source/BuiltInBasicType.swift
+++ b/Source/BuiltInBasicType.swift
@@ -136,7 +136,8 @@ extension String: _BuiltInBasicType {
                     return "false"
                 }
             }
-            return formatter.string(from: num)
+            let number = num as NSNumber
+            return number.stringValue
         case _ as NSNull:
             return nil
         default: