The boundary slip error resulting from the interpolation/spreading non-reciprocity of the direct-forcing immersedboundary method is analyzed based on a simple and generic theoretical framework. In explicit implementations, the slip error scales with the Courant number, as predicted by the analysis and confirmed by lattice-Boltzmann simulation results. Using an analytical approximation of the non-reciprocity error, the immersed-boundary force can be corrected in order to prevent boundary slip and flow penetration. This a priori correction leads to a major improvement of the no-slip condition while avoiding any additional computational time or implementation effort.