action with no side effect error

Using stateflow with matlab R2015b.

In the first state that is connected to the "default transition" arrow, I tried to use a simple if statement as shown below

IDLE
entry:
if(a==0)
 b=1;
else
 b=0;
end

Compiler is indicating that this is an error as follows:

else
^^^^^
'IDLE' has an action with no side effect.

Occurred in chart 'testSubsystem/testChart', 'IDLE'. [2 similar]
Component: Stateflow | Category: Parse warning
Unresolved data 'else' in 'IDLE'.
Component: Stateflow | Category: General error
Unresolved data 'end' in 'IDLE'.
Component: Stateflow | Category: General error
Unresolved function 'if' in 'IDLE'.
Component: Stateflow | Category: General errorOpen
Parsing failed for machine: 'test'
Component: Simulink | Category: Model error

I'm seriously confused why a simple if statement is not compiling right. I have attached the statflow project to this question for good measure Any help would be highly appreciated, Amish

[Tania Izquierdo]

How had you fixed it?