diff --git a/src/_zkapauthorizer/tahoe.py b/src/_zkapauthorizer/tahoe.py
index 848d51635aea7e602a2a287f4f3ae0ed49679b82..d9271d042446b4302e9e4cd745321d0f8304b932 100644
--- a/src/_zkapauthorizer/tahoe.py
+++ b/src/_zkapauthorizer/tahoe.py
@@ -59,7 +59,7 @@ def _scrub_cap(cap: str) -> str:
     can usually be distinguished from the scrubbed version of a different
     input string.
     """
-    scrubbed = b32encode(sha256(cap.encode("ascii")).digest())[:6]
+    scrubbed = b32encode(sha256(cap.encode("ascii")).digest())[:6].lower()
     return f"URI:SCRUBBED:{scrubbed}"