diff --git a/PaymentKit/PTKAddressZip.m b/PaymentKit/PTKAddressZip.m
index 1158cd9..0ffac76 100644
--- a/PaymentKit/PTKAddressZip.m
+++ b/PaymentKit/PTKAddressZip.m
@@ -35,7 +35,7 @@ - (BOOL)isValid
                                                             options:NSRegularExpressionSearch
                                                               range:NSMakeRange(0, _zip.length)];
 
-    return stripped.length > 2;
+    return stripped.length > 2 && stripped.length < 10;
 }
 
 - (BOOL)isPartiallyValid