Error when applying force to a rigidbody in planetary gravity simulation

Hi,

I'm implementing a point gravity system (planetary physics). In the script that revolves around an object (a moon, for example) I have:

private void Vector3 gravityVector;
void FixedUpdate() {

if (gravityIgnoresMass) {
GetComponent<Rigidbody>().AddForce(gravityVector, ForceMode.Acceleration);
}
else {
GetComponent<Rigidbody>().AddForce(gravityVector);
}

//clear for next round
gravityVector = Vector3.zero;
}


I'm getting the error: "rigidbody.force assign attempt for 'Sphere' is not valid. Input force is {NaN, NaN, NaN}."

So something's not right with gravityVector as the import force shows three objects that are not numbers.

What am I doing wrong and how do I fix it?

Thanks so much for the help!