Search option for labels doesn't work

The search option for labels ('l=<label-text>') is not working. Tested on Confluence 5.6.1. The problem seems to be related to the way the LabelsSearchFilter, created in ISearchHack, is transformed by Confluence to the internal representation of the search query. It is not clear at this point if other Confluence versions behave the same way. It is also not clear if the problem can be avoided by using the Search V2 API in a different way. Perhaps it is possible to add a LabelQuery to the chained query instead of using a LabelsSearchFilter.