diff --git a/mercurial/state.py b/mercurial/state.py
--- a/mercurial/state.py
+++ b/mercurial/state.py
@@ -55,7 +55,11 @@
         return self.exists()
 
     def __getitem__(self, key):
-        return self.opts[key]
+        try:
+            return self.opts[key]
+        except KeyError:
+            raise error.ProgrammingError(_("key '%s' not found in state"
+                                           " file %s") % (key, self.path))
 
     def load(self):
         """load the existing evolvestate file into the class object"""