Invalid Distance argument, must be a character string (kDtreeSearcher)

I am working on 3D points cloud with Matlab, and I want to compute the minimum distance of each point on an object X to all points on an other object Y.

To achieve this, I have constructed a kDtree for X and defined my own distance function Dist6D.m. Then using the knnsearch algorithm, I do this:

[match mindist] = knnsearch(kdOBJ,transpose(Y),'Distance',@Dist6D);

The problem I encounter is that it redirects me to the file

toolbox/stats/stats/KDTreeSearcher/knnsearch

and I can't define my own function. The only permitted functions are defined as strings and it doesn't allow me to use my own. in the description they wrote this :

 %    'Distance'     A string specifying the distance metric. The value can
%                   be one of the following:
%                     'euclidean'   - Euclidean distance
%                     'cityblock'   - City Block distance
%                     'chebychev'   - Chebychev distance (maximum
%                                     coordinate difference)
%                     'minkowski'   - Minkowski distance
%                     Default is NS.Distance  end

Lastly, I am obliged to use a kDtreeObject (Because using just X and Y without transforming one oh them to a Kdtree Obj solves the problem for me).