Optical add-drop multiplexers (OADMs) based on grating-assisted null couplers are excellent candidates for low-cost wavelength division multiplexed (WDM) systems. These devices potentially offer exceptional operating characteristics in a compact layout that does not require circulators. Optimization analysis has been already presented. However, inherent fabrication errors either in the null coupler or in the grating increase the crosstalk at the drop port. In this paper, a simple model that relates the optical signal-to-noise ratio (OSNR) of the OADM to the null-coupler and grating parameters is presented. Based on that model, we propose two new approaches to increase the OSNR at the drop at output ports, when fabrication errors are present. One approach optimizes the grating angle to minimize the noise produced by spurious reflection without mode conversion. The other shifts these reflections out of the bandwidth used by the WDM system. The advantages and limitations of these approaches are analyzed.