gistfile1.txt

diff --git a/master/buildbot/sourcestamp.py b/master/buildbot/sourcestamp.py
index dd78e44..aca6067 100644
--- a/master/buildbot/sourcestamp.py
+++ b/master/buildbot/sourcestamp.py
@@ -108,7 +108,7 @@ class SourceStamp(util.ComparableMixin, styles.Versioned):
         return d
 
     def __init__(self, branch=None, revision=None, patch=None,
-                 changes=None, project='', repository='', fromSsdict=False):
+                 changes=None, project=None, repository=None, fromSsdict=False):
         # skip all this madness if we're being built from the database
         if fromSsdict:
             return
@@ -117,9 +117,9 @@ class SourceStamp(util.ComparableMixin, styles.Versioned):
             assert len(patch) == 2
             assert int(patch[0]) != -1
         self.branch = branch
-        self.patch = patch
-        self.project = project
-        self.repository = repository
+        self.patch = patch or None
+        self.project = project or ''
+        self.repository = repository or ''
         if changes:
             self.changes = tuple(changes)
             # set branch and revision to most recent change