Why is my Unity C# function returning NaN values?

I am trying to calculate gravity between bodies using the following script:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Gravity : MonoBehaviour
{
    public const float gravity = 10;
    public float mass = 10f;
    public bool active = true;
    public Rigidbody rigidbody;
    public Transform transform;
    GameObject[] otherObjs;
    Gravity[] otherGravities;

    void Start () {
        rigidbody = gameObject.GetComponent<Rigidbody>();
        transform = gameObject.transform;
        otherObjs = GameObject.FindGameObjectsWithTag("Body");
        otherGravities = new Gravity[otherObjs.Length];
        for (int i = 0; i < otherObjs.Length; i++)
        {
            otherGravities[i] = otherObjs[i].GetComponent<Gravity>();
        }
    }

    void FixedUpdate () {
        rigidbody.AddForce(CalculateGravity());
    }

    Vector3 CalculateGravity () {
        Vector3 finalGravity = new Vector3(0, 0, 0);
        foreach (Gravity current in otherGravities) {
            float distance = Vector3.Distance(transform.position, current.transform.position);
            Vector3 unNormalDirection = transform.position - current.transform.position;
            Vector3 direction = unNormalDirection.normalized;
            Vector3 gravitationalPull = ((mass * current.mass) / (distance * distance)) * direction;
            finalGravity += gravitationalPull;
        }
        return finalGravity;
    }
}

When running the code, I get the following error

rigidbody.force assign attempt for 'Sun' is not valid. Input force is { NaN, NaN, NaN }.
UnityEngine.Rigidbody:AddForce (UnityEngine.Vector3)
Gravity:FixedUpdate () (at Assets/Scripts/Gravity.cs:27)

I get this for both objects in the scene.
I can't seem to figure out why the function is returning NaN's, I've double checked that it is not dividing by zero. If you have any idea, please let me know.

I'm basing this on a few assumptions. It looks like you're probably calculating gravitational attraction to bodies from themselves, with a distance of 0, and dividing by that 0.

In the line otherObjs = GameObject.FindGameObjectsWithTag("Body"); you're getting all objects with the tag "Body" which probably includes objects you're applying gravity to. I'd try to filter them something like this:

    void Start () {
        rigidbody = gameObject.GetComponent<Rigidbody>();
        transform = gameObject.transform;
        otherObjs = GameObject.FindGameObjectsWithTag("Body");
        int otherGravitiesLength = otherObjs.Length;
        if (gameObject.CompareTag("Body")) {
            //Must exclude this object from its own other gravities list.
            //So don't include it in the length of that array.
            otherGravitiesLength = otherGravitiesLength - 1;
        }
        otherGravities = new Gravity[otherGravitiesLength];
        int insertIndex = 0;
        for (int i = 0; i < otherObjs.Length; i++)
        {
            if (otherObjs[i] != gameObject) {
                otherGravities[insertIndex++] = otherObjs[i].GetComponent<Gravity>();
            }
        }
    }

I've never used Unity, so I'm making some guesses about the API from context, and a tiny bit of googling, so this answer may need some repair.

If it is possible for two distinct bodies that gravitationally attract to be in the same position, then you should probably still also test for a zero distance and handle that differently within CalculateGravity