Delphi dynamically calling different functions

type
  TNodeFunction = function(AInput: String): String;

implementation

function Func1(AInput: String): String;
begin
   result := AInput;
end;

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Then I can do this:
var
  SomeFunc,: TNodeFunction;
begin
     SomeFunc = Func1;
    ....
So far it is good.

But my problem is that a certain place I need to detect in my program that I gave value to SomeFunc or not.

I tried this :
If SomeFunc<> nil Then <- but I get this error [dcc32 Error]  E2008 Incompatible types

Thank you very much for your help!