diff --git a/lib/types/customers.d.ts b/lib/types/customers.d.ts
index c18d5c6..ebf39fe 100644
--- a/lib/types/customers.d.ts
+++ b/lib/types/customers.d.ts
@@ -25,7 +25,7 @@ export declare namespace Customers {
          *
          * `1` (default): If a customer with the same details already exists, throws an error.
          */
-        fail_existing?: boolean | 0 | 1;
+        fail_existing?: boolean | 0 | 1 | '0' | '1';
         /**
          * Customer's GST number, if available
          */