Compressor running with heat on...

[Chrispbrown36]

So I got motivated Friday and got the a/c up and going on the old truck. It cooled nicely but some of the duct work was off and I knew I had to change the fan switch so I tore into the dash today. After pulling it apart I was checking to make sure the blend door was closing all the way when I noticed that the a/c compressor would continue to run even after I had changed it over to heat or defrost. What might be causing this?