MATLAB simulink , mask subsystems, how to get the value in popup controller?

I use simulink to create a simple system, as this:

Note that the subsystem only contains a Gain modle , the subsystem is masked with "mask subsystem".

Then I set the parameter of Gain modle in mask editor as this:

when I chose 0.2 for a, the amplitude for sin wave should be 0.2, however , when I choose 0.2, the amplitude for sin wave is 2, the result of the scope is :

It seems that when I chose 0.2, I the true value of a is its index, the problem is how can I get the value in the popup controller?