diff --git a/lib/mbtiles.js b/lib/mbtiles.js
index 66f99a1..054f052 100644
--- a/lib/mbtiles.js
+++ b/lib/mbtiles.js
@@ -549,7 +549,11 @@ MBTiles.prototype.putTile = function(z, x, y, data, callback) {
 
     // Tilelive may pass us a data.key. If not, generate an md5
     // from the image buffer data.
-    var id = data.key || crypto.createHash('md5').update(data).digest('hex');
+    var id;
+    if (data.key)
+        id = String(data.key);
+    else
+        id = crypto.createHash('md5').update(data).digest('hex');
     if (!this._tileCache[id]) {
         // This corresponds to the images table.
         this._tileCache[id] = data;