Why a code doesn't work when running a Function but it works by F9 key?

Hello There,

I have written a function that contains the following code:

f=-fnval(spline,[nodes.x(nodes.topnodes) nodes.y(nodes.topnodes)]');

when I run the function, it always gets stuck in this line although everything seems to be fine. More interestingly, I set a breakpoint just before this line and run the function down to the breakpoint then run this line by F9 key and it works - but when running the whole function, it stops at this line. Anyone has an idea why?

Regards Salah