Binary quadratic form $f$ represents at least one integer relatively prime to a given integer $M$

Question

I encountered this statement on a reading on quadratic binary form.

"Let $f(x, y)$ be an arbitrary integral quadratic form, and let $M$ be an integer. Then $f$ represents at least one integer that is relatively prime to $M$."

I can see why this statement is true when $f$ is a primitive form, but what about when f is any form?