Expression code not saving

I'm trying to create an expression that simply takes a number (called 'shift') and returns a string like the following:

"This Shift ([shift])"

I tried creating an expression that takes 'shift' as an input and the expression code is simply:

"This Shift (" + shift + ")"

However, every time I try to create this expression, the expression code is empty the next time I open it and it doesn't actually work.