Undefined function macros canceling function

Hi there! So I tried using function macros and I noticed that for some reason if I run a function with an entity (for me I ran function ... with entity @s data as a marker) and if i try to use a user-defined string in the function ($(Sender); $(Recipient)), and that data does not exist or only one of them is given, the whole function just doesn't run. Is there a way around this, or am I doing something wrong?