diff --git a/src/_zkapauthorizer/_storage_server.py b/src/_zkapauthorizer/_storage_server.py
index c107525f50f34548f9a7cbcbe41ac066f64b244a..b78a7c4f4fde0c06d2e25ebdfa54519cedd79165 100644
--- a/src/_zkapauthorizer/_storage_server.py
+++ b/src/_zkapauthorizer/_storage_server.py
@@ -65,6 +65,12 @@ try:
 except ImportError:
     pass
 
+# Hack around a bug in prometheus_client
+def _prometheus_client_fix():
+    import prometheus_client.exposition
+    prometheus_client.exposition.FileNotFoundError = IOError
+_prometheus_client_fix()
+
 # See allmydata/storage/mutable.py
 SLOT_HEADER_SIZE = 468
 LEASE_TRAILER_SIZE = 4