diff --git a/src/pretix/base/models/orders.py b/src/pretix/base/models/orders.py
index d6ed86327..5193dbdee 100644
--- a/src/pretix/base/models/orders.py
+++ b/src/pretix/base/models/orders.py
@@ -17,7 +17,7 @@ from .items import Item, ItemVariation, Question, QuestionOption, Quota
 
 
 def generate_secret():
-    return get_random_string(length=16, allowed_chars=string.ascii_letters + string.digits)
+    return get_random_string(length=16, allowed_chars=string.ascii_lowercase + string.digits)
 
 
 def generate_position_secret():