In the decarboxylation of 2-methyl-2-vinyl malonate (2a) the wildtype enzyme produced (R)-2-methylbut-3-enoic acid ((R)-2b)) with only 96% ee. Furthermore, we were surprised to find that the (S)-selective variant AMDase V43I/G74C/A125P/V156L/ M159L/C188G (henceforth "AMDase ICPLLG") produced (S)-2b in a low optical purity of 66% ee (Scheme S1). To our knowledge, this is the first time that AMDase shows low stereoselectivity towards a disubstituted malonate. If AMDase ICPLLG followed the same mechanism as towards the aromatic 1a, the expected outcome would have been the pure (S)-product. The formation of both enantiomers by an AMDase mutant having only one catalytic cysteine in the active site (C74) is not compatible with our current understanding of the mechanism via stepwise pro-R decarboxylation followed by selective protonation from the re face.