Mudbox :: Map Extraction With Multiple Objects In One Map

I'm trying to extract a vector displacement map out of three objects.

All three of them are on the same UV tile, with no overlap between each other.

All three of them have the same subdivision level and share the same Mudbox material.

I choose "Generate one map for all targets".

When hitting extract, Mudbox tells me that :

"Subdivision based map extraction is available only when the number of source and target meshes are the same".

Well its the case. With the same order in target and source models (A-B-C)