Angular - assign custom validator to a FormGroup

I need to assign a custom validator to a FormGroup. I can do this at the time the FormGroup is created like this:

let myForm : FormGroup;

myForm = this.formBuilder.group({
myControl1: defaultValue,
myControl2: defaultValue
}, { validator: this.comparisonValidator })

comparisonValidator(g: FormGroup) {
if (g.get('myControl1').value > g.get('myControl2'.value)) {
g.controls['myControl1'].setErrors({ 'value2GreaterThanValue1': true });
return;
}
}


I have a situation though where I need to add the custom validator after I've instantiated the FormGroup, so I'm trying to do this after instantiating myForm, instead of adding the validator when the form is instantiated:

let myForm : FormGroup;

myForm = this.formBuilder.group({
myControl1: defaultValue,
myControl2: defaultValue
})

this.myForm.validator = this.comparisonValidator;


This gives me a compiler error:

Type '(g: FormGroup) => void' is not assignable to type 'ValidatorFn'.
Type 'void' is not assignable to type 'ValidationErrors'.


How do I assign a validator to my FormGroup so that the formGroup is passed as the argument to my comparisonValidator function?

Update - I've added a line showing where I'm doing a setErrors in my comparisonValidator, to make it clearer exactly how I'm trying to set a validation error.

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