problems with EL function calls

[Peter Ryan <pryan@xxxxxxxxxxxxxx>]

On many occasions while calling functions i have written myself, I get an error message
	"arrays and variables are not allowed here".

Typically this happens when I do something like:
val = FastK(length)
funcval = myfunc(val)

I have to correct it to:
funcval = myfunc(FastK(length))

Can someone explain to me why this is, and what options I have to avoid it.
It drives me mad.

Thanks