SAS Call Symput within do loop and if statement not working

I need to define a variable called test_var based on what the input is to a macro. Here is a simplified version of what I'm trying to achieve in SAS:

%macro testing(blah);

    %if &blah. = 1 %then %do;
    data _null_;
        call symput('test_var',"a1");
    run;
    %end;
%mend;

%testing(1);
%put &test_var.;

But i get the error: WARNING: Apparent symbolic reference TEST_VAR not resolved.