unity 5.6 assetbundle shader name lost

hi there, im using Unity 5.1.0f1 version. Now i decide to upgrade Unity 5.6, when i load assetbundles which builded in Unity5.1.0f3 ,i found the shader name is empty,so i cant use "obj.material.shader=Shader.Find("XXX")" to reset the shader. So, is there any way to get the shader name in Unity 5.6? I dont want to rebuild all the assetbundles in 5.6.