diff --git a/lib/mbtiles.js b/lib/mbtiles.js
index 1bfa462..41f1255 100644
--- a/lib/mbtiles.js
+++ b/lib/mbtiles.js
@@ -588,17 +588,17 @@ MBTiles.prototype.putGrid = function(z, x, y, data, callback) {
         /*
         this._gridCache[id] = zlib.deflate(new Buffer(json, 'utf8'));
         */
-        
+        var that = this;
         Step(function() {
             zlib.deflate(new Buffer(json, 'utf8'),this);
         }, function(err,buffer) {
             if (err) return callback(err);
-            this._gridCache[id] = buffer;
+            that._gridCache[id] = buffer;
             // This corresponds to the grid_key table.
-            this._keyCache[id] = Object.keys(data.data || {});
+            that._keyCache[id] = Object.keys(data.data || {});
     
             // This corresponds to the keymap table.
-            _(this._dataCache).extend(data.data || {});
+            _(that._dataCache).extend(data.data || {});
         }
         );
     }