I am wondering if I'm doing something wrong?

I've been using segwit P2SH-P2WPKH address for the last year.
I went to change my payout address on a mining pool, Slush and to do that.  It is merely signing a message to unlock that option.  When I went into armory to sign their message from that address I got a pop up message saying:

  You are attempting to sign a message with a P2SH address.  This feature is not supported at the moment.

then next pop up saying:

  The private key is not known for this address.

I hadn't changed the pool payout address since I started using the segwit vs. legacy addresses.  With Legacy I had always changed addresses each time I spent. The segwit address spent just fine but don't really want to keep funding it with mining.  So I am wondering if I'm doing something wrong.