@PageMark and @Tbl width=expand option not playing nicely together?

For some reason, when I try to use @PageMark prior to a table with
width=expand, I'm getting an error. Sample program:

@SysInclude { tbl }
@SysInclude { doc }

@Doc @Text @Begin

@PageMark sig.table

@Tbl
  width {expand}
  aformat { @Cell A | @Cell B }
{

@Rowa
  A {a}
  B {a}
}


@End @Text

Comment out the @PageMark and it runs fine. Comment out the width option
and it works fine. However, with the @PageMark and width=expand in there
I'm getting the result:

 6,1: 80.000c object deleted (too wide for 15.989c paragraph)


I don't understand why using a @PageMark should effect the output so much.
Why would it just arbritrarily expand the table to 80 cm wide??