rosservice call /mavros/set_mode "base_mode: 0 custom_mode: 'OFFBOARD'

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I am trying to change the mode of my iris drone in gazebo using ROS Service from its base mode MANUAL to custom mode OFFBOARD. But when I hit the following command in the terminal, I get the following error. What should I do to resolve this issue

$ rosservice call /mavros/set_mode manual offboard

ERROR: Unable to send request. One of the fields has an incorrect type:
field base_mode must be unsigned integer type

srv file: uint8 MAV_MODE_PREFLIGHT=0 uint8 MAV_MODE_STABILIZE_DISARMED=80 uint8 MAV_MODE_STABILIZE_ARMED=208 uint8 MAV_MODE_MANUAL_DISARMED=64 uint8 MAV_MODE_MANUAL_ARMED=192 uint8 MAV_MODE_GUIDED_DISARMED=88 uint8 MAV_MODE_GUIDED_ARMED=216 uint8 MAV_MODE_AUTO_DISARMED=92 uint8 MAV_MODE_AUTO_ARMED=220 uint8 MAV_MODE_TEST_DISARMED=66 uint8 MAV_MODE_TEST_ARMED=194 uint8 base_mode string custom_mode --- bool mode_sent