diff --git a/src/MappedCollection2.coffee b/src/MappedCollection2.coffee
index 6c1eecce3..be79e95ff 100644
--- a/src/MappedCollection2.coffee
+++ b/src/MappedCollection2.coffee
@@ -311,7 +311,7 @@ class $MappedCollection2 extends $EventEmitter
         if not $_.isObject def
           parent[prop] = def
         else if $_.isFunction def
-          parent[prop] = def.call proxy
+          parent[prop] = def.call proxy, parent[prop]
         else if $_.isArray def
           i = 0
           n = def.length