Can't compile extension method in a plugin.

i use a dll as plugin in my project, it works well. but, when i add a extension method in the dll, unity tell me "Internal compiler error. See the console log for more information. output was: Unhandled Exception: System.Reflection.ReflectionTypeLoadException: The classes in the module cannot be loaded." . why other dll can use extension method, but my compiler failed. please help me.