Mass measurements with the ISOLTRAP mass spectrometer at CERN-ISOLDE improve mass uncertainties of neutron-deficient tin isotopes towards doubly magic . The mass uncertainty of was reduced by a factor of 4, and the new value for the mass excess of is compared with nuclear ab initio and density functional theory calculations. Based on these results and local trends in the mass surface, the masses of , as determined through their values, were found to be inconsistent with the new results. From our measurement for , we extrapolate the mass excess of to , which is significantly more bound than previously suggested. By correcting the mass values for , we also adjust the values of , and near the proton drip line which are connected through their and proton values. The results show an overall smoothening of the mass surface, suggesting the absence of deformation energy above the shell closure.