Breaking When Condition is True

[SodaPop]

I would like the debugger to break on a particular line when an expression is true. Whenever I try this, I get a message "Error in Expression." The expression I'm currently using is

strlen(buffer)>0

However, I get this message with almost any other expression. I also receive this message in the Watch tab.

What is the proper syntax for writing debugger expressions?

Thanks.