Decrease the gravity in half for 10 seconds

void gravdown()
{
StartCoroutine(downgrav());

    }

    private IEnumerator downgrav()
    {
        //codes
    }

I have a button. And when this button is pressed, the current gravity value has to be halved for a time like 10 seconds and then return to its original state. When I did some research on how to do this, I came across coroutine. But “IEnumerator downgrav ()” I get an error in this part. CS0161 ‘’: not all code paths return a value coroutine ". Since it’s something I’ve never used before, I don’t know what to do. How can I do what I want?

private IEnumerator downgrav ()
{
Vector3 initialGravity = Physics.gravity;
Physics.gravity = initialGravity * 0.5f;
yield return new WaitForSeconds( 10f );
Physics.gravity = initialGravity;
}