Dusty star-forming galaxies (DSFGs) detected at z > 4 provide important examples of the first generations of massive galaxies. However, few examples with spectroscopic confirmation are currently known, with Hershel struggling to detect significant numbers of z > 6 DSFGs. NGP6_D1 is a bright 850 μm source (12.3 ± 2.5 mJy) with no counterparts at shorter wavelengths (a SPIRE dropout). Interferometric observations confirm it is a single source, with no evidence for any optical or NIR emission, or nearby likely foreground lensing sources. No >3σ detected lines are seen in both LMT Redshift Search Receiver and IRAM 30 m EMIR spectra of NGP6_D1 across 32 GHz of bandwidth despite reaching detection limits of ̃ 1 mJy/500 km s^{-1}, so the redshift remains unknown. Template fitting suggests that NGP6_D1 is most likely between z = 5.8 and 8.3. SED analysis finds that NGP6_D1 is a ULIRG, with a dust mass ̃108-109 M☉ and a star-formation rate of ̃500 M☉ yr-1. We place upper limits on the gas mass of NGP6_D1 of MH2 <(1.1 ± 3.5) × 1011 M☉, consistent with a gas-to-dust ratio of ̃100-1000. We discuss the nature of NGP6_D1 in the context of the broader sub-mm population, and find that comparable SPIRE dropouts account for ̃20 per cent of all SCUBA-2 detected sources, but with a similar flux density distribution to the general population.