Syntax error due to empty string or sequence in xdmp:unpath

[joecrean]

Trying to merge some documents using xquery in query console.

The call to merging:build-merge-models-by-uri results in the following exception

XDMP-UNEXPECTED: (err:XPST0003) Unexpected token syntax error, unexpected $end

Specifically In /com.marklogic.smart-mastering/survivorship/merging/base.xqy on line 1761

It boils down to passing an empty string to xdmp:unpath in the function get-instance-props-by-path.
Here is the link to the line of code - the variable middle-path is empty.

smart-mastering-core/src/main/ml-modules/root/com.marklogic.smart-mastering/survivorship/merging/base.xqy

Line 1586 in c77acfe

xdmp:unpath($middle-path, $ns-map, $instance)/node()[fn:node-name(.) = $target-name]

In my case i have a simple Entity structure in a standard envelope structure (DHF 411).
e.g. /es:envelope/es:instance/FooEntity/keyword. The string-join call on 1578 excludes the last
member of the tokenised sequence and thus the only member of that sequence in my case thus resulting in an empty sequence which is passed into the unpath call and at that point throws
the above exception.