diff --git a/webagg/app.py b/webagg/app.py
index cb5e8bb0..38177fa0 100644
--- a/webagg/app.py
+++ b/webagg/app.py
@@ -74,6 +74,8 @@ class ResAggApp(object):
                 res = self.json_encode(res, out_headers)
 
             if errs:
+                if 'last_exc' in errs:
+                    errs['last_exc'] = str(errs['last_exc'])
                 out_headers['ResErrors'] = json.dumps(errs)
 
             start_response('200 OK', list(out_headers.items()))
diff --git a/webagg/responseloader.py b/webagg/responseloader.py
index 00f01069..5a8326b0 100644
--- a/webagg/responseloader.py
+++ b/webagg/responseloader.py
@@ -187,7 +187,8 @@ class WARCPathLoader(BaseLoader):
         if not cdx.get('filename') or cdx.get('offset') is None:
             return None
 
-        cdx._formatter = ParamFormatter(params, cdx.get('source'))
+        orig_source = cdx.get('source', '').split(':')[0]
+        cdx._formatter = ParamFormatter(params, orig_source)
 
         failed_files = []
         headers, payload = (self.resolve_loader.