Unrecognized assets cannot be included in AssetBundles

I want to update a old project(running in unity 4.x)to unity 5,when using assets bundle,the old prefab can not be packed in the bundle,all the information is:

Unrecognized assets cannot be included in AssetBundles: "Assets/StreamingAssets/_prefabs/streaming_selavatar_Main_Camera.120.prefab".
UnityEditor.BuildPipeline:BuildAssetBundles(String, AssetBundleBuild[], BuildAssetBundleOptions, BuildTarget)
ExportAssetBundles:ExportResourceAutoAll() (at Assets/Editor/ExprotAssetBundles.cs:576)

the prefab can not be packed into the bundle,can anyone tell me what i should do to solve it?
thx a lot!

the address of the project:git@git.oschina.net:firesuiry/kbengine_unity3d_5_x_x_warring-master.git

i found the prefab is unrecognized too…

sorry to find that the problem may be caused by other problems sorry…

Maybe it’s because the folder starts with _ ?