Unexpected end-group tag.

Raise code

  def MergeFromString(self, serialized):
    serialized = memoryview(serialized)
    length = len(serialized)
    try:
      if self._InternalParse(serialized, 0, length) != length:
        # The only reason _InternalParse would return early is if it
        # encountered an end-group tag.
        raise message_mod.DecodeError('Unexpected end-group tag.')
    except (IndexError, TypeError):
      # Now ord(buf[p:p+1]) == ord('') gets TypeError.
      raise message_mod.DecodeError('Truncated message.')
    except struct.error as e:
      raise message_mod.DecodeError(e)
    return length   # Return this for legacy reasons.
  cls.MergeFromString = MergeFromString

Comment explaining raise

The only reason _InternalParse would return early is if it encountered an end-group tag.