[问]作sweep feature时遇到的不懂之处

我画了一条不封闭的曲线作为trajectory,但是勾选完成时pro-e显示提示信息如下:
Select a perpendicular surface, an edge, or vertex relative to which the section will be dimensined and constrained.

曲线在Front面,在该面内已constrain. 当我按照提示点选Top面作为perpendicular surface时,系统又提示:
Section must contain geometric entities for this feature.
勾选失败.
请问我应如果操作.