diff --git a/CHANGES b/CHANGES
index 07b4c314a..43da376af 100644
--- a/CHANGES
+++ b/CHANGES
@@ -16,6 +16,7 @@ Bugs fixed
 * PR#311: sphinx-quickstart does not work on python 3.4.
 * Fix :confval:`autodoc_docstring_signature` not working with signatures
   in class docstrings.
+* Rebuilding cause crash unexpectedly when source files were added.
 
 
 Release 1.3b1 (released Oct 10, 2014)
diff --git a/sphinx/builders/__init__.py b/sphinx/builders/__init__.py
index 64ae2a098..70ec6cb56 100644
--- a/sphinx/builders/__init__.py
+++ b/sphinx/builders/__init__.py
@@ -244,7 +244,6 @@ class Builder(object):
         if summary:
             self.info(bold('building [%s]' % self.name) + ': ' + summary)
 
-        updated_docnames = set()
         # while reading, collect all warnings from docutils
         warnings = []
         self.env.set_warnfunc(lambda *args: warnings.append(args))
diff --git a/sphinx/environment.py b/sphinx/environment.py
index c3d67d733..423273541 100644
--- a/sphinx/environment.py
+++ b/sphinx/environment.py
@@ -590,7 +590,7 @@ class BuildEnvironment:
 
         self.app = None
         app.emit('env-updated', self)
-        return docnames
+        return set(docnames)
 
     def _read_serial(self, docnames, app):
         for docname in app.status_iterator(docnames, 'reading sources... ',