This paper introduces a sensor system and motion that measure and compensate the error in position and orientation between robot's end-effectors and the rungs of ladder while climbing a ladder. This error has been a threat to vertical ladder climbing for a four-limbed robot and may directly cause failure in climbing, thus must be compensated. In detail, our error compensation system consists of 8 proximity sensors (2 sensors for each end-effector) powered by lithium batteries and data is transmitted via wireless communication. With the system constructed, corresponding algorithms to measure and calculate the amount of error in position and orientation as well as motion planning of the robot to compensate the error are proposed. Additionally, countermeasures are also prepared to deal with undesired situations, such as communication in low quality and incorrect data returned from proximity sensors. Simulation results of comparison with and without error compensation are presented and experiment results of the real robot are given to validate the effectiveness of our proposed system. Finally, discussions about results are shown and expected prospective works are concluded.