A constructor call to superclass appears after a return

I am trying to check if a parameter is of a certain type when instantiating a subclass of a superclass, and I keep getting an error that appears to be associated with my method of checking of the argument type. The debugger won't even let me include a breakpoint without throwing the error "A constructor call to superclass appears after the object is used, or after a return." I think it's pretty obvious what line of code breaks my classes, but why am I not allowed to do this type checking? What other ways are there to confirm that my arguments are of a particular type? Code below.

classdef superClass < handle
  properties
      PropertyOne
      PropertyTwo
  end    
  methods
      function sup = superClass(param1, param2)
          sup.PropertyOne = param1;
          sup.PropertyTwo = param2;
      end
  end
end
classdef subClass < superClass
  properties
      PropertyThree
  end   
  methods
      function sub = subClass(param1, param2, param3)
          if ~isa(param1, 'char')
              disp('param1 must be type char')
              return
          end
          sub@superClass(param1, param2);
          sub.PropertyThree = param3;        
      end
  end
end

Answer 1

[Philip Borghesani]

Call error instead of disp and return. The constructor must return a valid object in sub when return is called and your code has not initialized it yet.

[Mark Duarte]

Thanks Phillip this worked. I will say though that if the disp and return were in the superclass, I am able to use the return statement in the constructor and it escapes without any error.