¿What's wrong with this? (getting the closest object in a 2d game)

     void GetClosestObject(string tag)
     {
         var objectsWithTag = GameObject.FindGameObjectsWithTag(tag);
         GameObject closestObject = null;
         foreach (GameObject obj in objectsWithTag)
         {
             if (closestObject == null)
             {
                 closestObject = obj;
             }
             else if (Vector2.Distance(transform.position, obj.transform.position) <= Vector2.Distance(transform.position, closestObject.transform.position))
             {
                 closestObject = obj;
             }
         }
         return closestObject;
     }
 }

It keeps returning me the following error.

 CS0127    Since 'IA.GetClosestObject(string)' returns void, a return keyword must not be followed by an object expression    StillAlive.CSharp    G:\toni\probes\StillAlive\Assets\IA.cs    167
 

I wonder why? I'm just looping through all GameObjects with an specific tag and then compare distances, storing the closest gameobject in a variable and then returning it.

Any help would be apreciated. Thanks,