The wedge calibration method is the most popular calibration technique in friction force microscopy for converting raw lateral laser deflection signals (in volt) into forces (in newton). Recent trends in nanotribology demand the use of the method at nanonewton (nN) force ranges; however, this method fails at these small forces. The objective of the present work is to identify the reason why the conventional wedge calibration method fails at nN force ranges. We found that the equation used in the model in this method amplifies experimental errors by orders of magnitude only at small setpoints purely due to its mathematical expression. This low tolerance against experimental errors in nanonewton force ranges is the reason for the failure. We identified that the condition, under which the method operates accurately, is adhesion ≪ setpoint. Discovery of this operation range (adhesion ≪ setpoint) is important because performing the calibration under other conditions can wrongly calibrate the system by orders of magnitude.