diff --git a/src/pretix/presale/checkoutflow.py b/src/pretix/presale/checkoutflow.py
index d43644f00d..6e97914d1d 100644
--- a/src/pretix/presale/checkoutflow.py
+++ b/src/pretix/presale/checkoutflow.py
@@ -709,8 +709,8 @@ class QuestionsStep(QuestionsViewMixin, CartMixin, TemplateFlowStep):
         if self.cart_customer:
             initial['email'] = self.cart_customer.email
             initial['email_repeat'] = self.cart_customer.email
-            if not initial['phone']:
-                initial['phone'] = str(self.cart_customer.phone)
+            if not initial['phone'] and self.cart_customer.phone:
+                initial['phone'] = self.cart_customer.phone
 
         f = ContactForm(data=self.request.POST if self.request.method == "POST" else None,
                         event=self.request.event,