diff --git a/src/couch_db.erl b/src/couch_db.erl
index 8005e6d..d01a3e0 100644
--- a/src/couch_db.erl
+++ b/src/couch_db.erl
@@ -391,7 +391,7 @@
         case couch_btree:size(T) of
             _ when Acc == null ->
                 null;
-            undefined ->
+            nil ->
                 null;
             Size ->
                 Acc + Size