This issue was originally reported in MQTT-417.
Line 2037-2039
"3) If the receiver does not already have a mapping for this Topic Alias
a) If the packet has a zero length Topic Name field it is a Protocol Error and the receiver uses DISCONNECT with Reason Code of 0x82 (Protocol Error) as described in section 4.13."
Why not use Return Code of 0x94 Topic Alias invalid?