Braces with TabStaff

[Richard Shann]

When I try to create a ChoirStaff brace around two staves one of which
is a TabStaff I get two braces:

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\version "2.20.0"
music = { c''1 }

\score { 
          <<
 \new ChoirStaff <<
                    \new TabStaff = "Part 1"  << 
                                 \music
                        >> 
                    \new Staff
                        << 
                           \new Voice { 
                                        \music
                                      } 
                        >> 
                 >>
          >>
       }
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If I replace TabStaff with Staff in the above, all is well.
Can anyone shed light on this?

Richard Shann