@Wouter: As Stas said, you might report that error. I don't agree with Laura who tried to under estimate that "syntax error". It's also about quality ;)
Don't get me wrong, I am all in favor of data quality! :) So if this can be fixed, it's better! The thing is, that I've seen so many datasets with these kind of type errors, that by now I pretty much live with them and I'm OK with these warnings (the triple is not broken after all, it's just not following the standards).
@Laura: Do you have a different rdf2hdt program or the one in the GitHub of HDT project ?
I just use https://github.com/rdfhdt/hdt-cpp compiled from the master branch. To verify data instead, I use RIOT (a CL tool from the Apache Jena package) like this `riot --validate file.nt`.