Compiler error C0234

Message: First Operand of __QueryInterface must be an interface reference or the instance of a function block

Possible error cause: Incorrect operands are passed to the operator ‎__QueryInterface‎.

Error correction: Pass an interface reference or the instance of a function block.

Example of the error:

PROGRAM PLC_PRG
VAR
	a : INT;
	ITFref, ITFref2 : ITF; 
	ITFref2 : ITF2;
END_VAR

__QueryInterface(a ,ITFref);

INTERFACE ITF EXTENDS __SYSTEM.IQueryInterface
INTERFACE ITF2 EXTENDS ITF

--> C0234: First Operand of __QueryInterface must be an interface reference or the instance of a function block

Error correction:

Example:
__QueryInterface(ITFref2, ITFref);