The usage of nillable

I assume that the nillable attribute be only applied to the text value of 
an element, e .g. <shipDate xsi:nil="true"/>, here shipDate only contains 
simpleContent. However, I couldn't find the approve for it.  Is it valid 
if applying it to a complexType element, e.g. <shipInfo xsi:nil="true"/>, 
here shipInfo contains a sequence of elements?

Jingkun