[Exist-open] XQuery bug?

[Hugh C. <hca...@em...>]

I'm trying to run the following query on a collection of METS/MODS docs:

declare namespace mods="http://www.loc.gov/mods/v3";
<result>
{
let $types := distinct-values(//mods:classification)
for $t in $types
let $c := count(//mods:mods[mods:classification = $t])
return <type>{$t} <num>{$c}</num></type>
}
</result>

This works in plain old Saxon 9 if I point it at one of my METS  
documents, but it breaks if I run it as an eXist query, with the  
message:

XPTY0018: Cannot mix nodes and atomic values in the result of a path  
expression.

Am I being dense?  Or is this a bug?  I'm running eXist 1.2.4.

Thanks,
Hugh

/**
  * Hugh A. Cayless, Ph.D
  * Head, Research & Development Group
  * Carolina Digital Library and Archives
  * UNC Chapel Hill
  * hca...@em...
  */