RuntimeError command

I am trying to build a mesh library and almost in every function I check for the existence of an argument

Function MyFunction(t.MyType, ...)

  if t = Null then RuntimeError "MyFunction: t does not exists!" : Return
.
.
.

End Function

I am wondering if this practice is a bottle neck to the execution speed. On the other hand I think it is not right to build a library without error check.