Pramaxo impurity C-D6 labelled. (6S)-N6-[3-[[(6S)-2-Amino-4,5,6,7-tetrahydro-6-benzothiazolyl]amino]-1-ethyl-2-methylpropyl]-4,5,6,7-tetrahydro-2,6-benzothiazolediamine-D6 can be byproduct of process for the preparation of pramipexole and salts thereof by reductive amination of (diamino)tetrahydrobenzothiazole with propanal.