When 2 buttons pressed animation stops?

Hey, So im doing a walking animation. I currently use this script to make it loop when holding the buttons down.

  #pragma strict

      function Update()

      {

      if(Input.GetKey(KeyCode.W))

      animation.Play("walk");

  if(Input.GetKey(KeyCode.S))

      animation.Play("walk");

  if(Input.GetKey(KeyCode.A))

      animation.Play("left");

  if(Input.GetKey(KeyCode.D))

      animation.Play("right");

      else if(!Input.anyKey)

  animation.Play("idle");

      }

 

However when i press 2 of the buttons down such as D & S It just freezes the animation and doesn't play anything?

This is obviously because the two animations are conflicting when trying to play but How can i get around this?

Thanks