The carbamate post-translational modification (PTM), formed by the nucleophilic attack of carbon dioxide by a dissociated lysine epsilon-amino group, is proposed as a widespread mechanism for sensing this biologically important bioactive gas. Here, we demonstrate the discovery and in vitro characterization of a carbamate PTM on K9 of nucleoside diphosphate kinase (NDK1). We demonstrate that altered side chain reactivity at K9 is deleterious for NDK1 structure and catalytic function, but that CO does not impact catalysis. We show that nucleotide substrate removes CO from NDK1, and the carbamate PTM is functionless within the detection limits of our experiments. The NDK1 K9 PTM is the first demonstration of a functionless carbamate. In light of this finding, we speculate that non-functionality is a possible feature of the many newly identified carbamate PTMs.