Dll Import

I got the following error while I put a dll file to plugins folder. Everything is working well except MonoBehaviour scripts are not visible in Component/Scripts. I have built dll with "Visual Studio and Unity 3.5 .net Subset Base Class Libraries" Target Framework in VS.

NullReferenceException: Object reference not set to an instance of an object
at UnityEditor.AssemblyHelper.IsTypeMonoBehaviourOrScriptableObject (Mono.Cecil.AssemblyDefinition assembly, Mono.Cecil.TypeReference type) [0x0008d] in C:\BuildAgent\work\d63dfc6385190b60\Editor\Mono\AssemblyHelper.cs:206

at UnityEditor.AssemblyHelper.ExtractAllClassesThatInheritMonoBehaviourAndScriptableObject (System.String path, System.String[]& classNamesArray, System.String[]& classNameSpacesArray) [0x00075] in C:\BuildAgent\work\d63dfc6385190b60\Editor\Mono\AssemblyHelper.cs:247