diff --git a/spec/index.md b/spec/index.md
index 872993169..05c05764f 100644
--- a/spec/index.md
+++ b/spec/index.md
@@ -3639,7 +3639,7 @@ S.messages = Older_messages · CallMessage M · Younger_messages
 (M.queue = Unordered) or (∀ msg ∈ Older_messages. msg.queue ≠ M.queue)
 M.method_name = 'upload_chunk'
 M.arg = candid(A)
-chunk_store_size = |chunk_store[M.canister_id]|
+chunk_store_size = |{x | chunk_store[M.canister_id][x] not null}|
 chunk_store_size < MAX_CHUNK_STORE_SIZE and (M.caller ∈ S.controllers[A.canister_id] ∪ {A.canister_id})
 hash = SHA-256(A.chunk)