Forward-port r91439.

src/ZODB/blob.py

@@ -120,6 +120,9 @@ class Blob(persistent.Persistent):
         if self.writers:
             raise BlobError("Already opened for writing.")
 
+        if self.readers is None:
+            self.readers = []
+
         if mode == 'r':
             if self._current_filename() is None:
                 self._create_uncommitted_file()

src/ZODB/tests/testblob.py

@@ -141,6 +141,10 @@ class BlobCloneTests(BlobTests):
         clone = u.load()
         clone._p_invalidate()
 
+        # it should also be possible to open the cloned blob
+        # (even though it won't contain the original data)
+        clone.open()
+
 
 class BlobUndoTests(BlobTests):