ReplaceString for brackets

Hi, I've been trying to replace a bracket in the sync rule, i.e.

ReplaceString(ReplaceString(someattribute,"(","#"),")","#")

but FIM is complaining the function ReplaceString is not correctly formatted.

Also I notice

If I use CustomExpression 

ReplaceString(someattribute,"(","#")

FIM fails with ReplaceString is not correctly formatted.

but if I select FIM function -> ReplaceString and enter oldstring as ) and newstring as #, FIM would happily accept.

Any idea how I can get around this?

Thanks,

John