Comparators connected to Command Blocks don't shut off after executing a command

Comparator connected to Command Block won't shut off after executing a single-use command.

Set-Up:
Place a Command Block, connect Comparator to it and type in simple command:
"say x"
And attach a button to the Command Block.
After pressing, command will be executed, comparator will lit up, but won't go out, even after button pops back.

The only thing that makes comparator go out is changing(or just pressing 'Done') command in the Command Block.