Can not find needed track (joint). problem

HELLO ! i have a problem in importing an animation only without skeletal mesh to unreal engine 4 in fbx format . but when i save my motion in fbx and import it to unreal engine 4 i selected Animation and choose the skeleton . when i press import or import all it give me a message that it cannot import that and give me an error like ( Cannot find needed track (joint). how can i import the fbx animation without any error especially this . thanks