diff --git a/src/buildstream/_cas/cascache.py b/src/buildstream/_cas/cascache.py
index 6a4043f73..fbfc150f3 100644
--- a/src/buildstream/_cas/cascache.py
+++ b/src/buildstream/_cas/cascache.py
@@ -702,7 +702,7 @@ def send_blobs(self, remote, digests):
         batch.send()
 
     def _send_directory(self, remote, digest):
-        required_blobs = self.required_blobs_for_directory(digest)
+        required_blobs = list(self.required_blobs_for_directory(digest))
 
         # Upload any blobs missing on the server.
         # buildbox-casd will call FindMissingBlobs before the actual upload