How to get IParagraphUnit from ISegmentPair

When I tried to get  IParagraphUnit from ISegmentPair.Source or  ISegmentPair.Target in order to get context information, IParagraphUnit was always NULL.

How can i get  IParagraphUnit from ISegmentPair.Source or ISegmentPair.Target ?

Here is my sample code.

var editorController = SdlTradosStudio.Application.GetController<EditorController>();
ISegment segment = editorController.ActiveDocument.ActiveSegmentPair.Source;
IParagraphUnit paraUnit = segment.ParentParagraphUnit;

IParagraphUnit is always NULL.

Any help appreciated

Best regards,

Masayuki