diff --git a/error.py b/error.py
index 029e122..6b9dff4 100644
--- a/error.py
+++ b/error.py
@@ -17,6 +17,10 @@
   """Failed to parse the manifest file.
   """
 
+class ManifestInvalidRevisionError(Exception):
+  """The revision value in a project is incorrect.
+  """
+
 class EditorError(Exception):
   """Unspecified error from the user's text editor.
   """