diff --git a/src/Gateway.php b/src/Gateway.php
index 66f4580..de14a85 100644
--- a/src/Gateway.php
+++ b/src/Gateway.php
@@ -174,6 +174,10 @@ public function start( Payment $payment ) {
 		// Purchase ID.
 		$purchase_id = $payment->format_string( (string) $this->config->get_purchase_id() );
 
+		if ( '' === $purchase_id ) {
+			$purchase_id = $payment->get_id();
+		}
+
 		$payment->set_meta( 'purchase_id', $purchase_id );
 
 		/**