Fixed x axis rotation

I try to make cannon aim at enemy.

The code work well for fixed y axis,
but when i replace "direction.y=0" by "direction.x=0" , something go wrong.

When the enemy is far away, it works well. However when the enemy move pass cannon in z direction, the cannon suddenly rotates in x axis and become upside down.

Code (CSharp):

1.    GameObject enemy = GameObject.Find("Enemy");
2.  
3.         Vector3 direction = enemy.transform.position - transform.position;
4.  
5.         direction.y = 0;
6.      
7.         Quaternion toRotation =  Quaternion.LookRotation(direction);
8.        
9.         transform.rotation = Quaternion.RotateTowards(transform.rotation, toRotation, 10 * Time.deltaTime);