diff --git a/src/serve_rendered.js b/src/serve_rendered.js
index 65860a8..6447b5e 100644
--- a/src/serve_rendered.js
+++ b/src/serve_rendered.js
@@ -169,7 +169,7 @@ module.exports = function(options, repo, params, id, dataResolver) {
     var source = styleJSON.sources[name];
     var url = source.url;
 
-    if (url.lastIndexOf('mbtiles:', 0) === 0) {
+    if (url && url.lastIndexOf('mbtiles:', 0) === 0) {
       // found mbtiles source, replace with info from local file
       delete source.url;
 
diff --git a/src/serve_style.js b/src/serve_style.js
index 71d7529..6613f33 100644
--- a/src/serve_style.js
+++ b/src/serve_style.js
@@ -16,7 +16,7 @@ module.exports = function(options, repo, params, id, reportTiles, reportFont) {
   Object.keys(styleJSON.sources).forEach(function(name) {
     var source = styleJSON.sources[name];
     var url = source.url;
-    if (url.lastIndexOf('mbtiles:', 0) === 0) {
+    if (url && url.lastIndexOf('mbtiles:', 0) === 0) {
       var mbtilesFile = url.substring('mbtiles://'.length);
       var fromData = mbtilesFile[0] == '{' &&
                      mbtilesFile[mbtilesFile.length - 1] == '}';