Error in robot control (gazebo) - load_parameters: unable to set parameters (last param was [/bl_wheel_trans_controller/pid/i=1.0]): cannot marshal None unless allow_none is enabled

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Hello, I'm trying to control my robot which is written in xacro file. I added launch file that is responsible for controlling the robot, which looks like this:

<param name="robot_decription" command="$(find xacro)/xacro '$(find mybot_description)/urdf/mybot.xacro' " />

<node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen" args="-urdf -model mybot -param robot_description -y -6"/>
<!-- load the controllers -->
<rosparam file="$(find mybot_control)/config/mybot_control.yaml" command="load"/>
<node name="controller_spawner" pkg ="controller_manager" type="spawner" ns="/mybot" args="bl_wheel_trans_controller br_wheel_trans_controller joint_state_controller --shutdown-timeout 3"/>
<!-- converts joint states to TF transforms -->
<node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher" respawn="false" output="screen">
    <remap from="joint_state" to="/mybot/joint_states" />
</node>

My config.yaml looks like this:

mybot:
joint_state_controller:
    type: joint_state_controller/JointStateController
    publish_rate: 20

bl_wheel_trans_controller:
    type: effort_controllers/JointPositionController
    joint: bl_wheel_hinge
    pid: {p: 1.0, i: 1.0, d: 0.0}

br_wheel_trans_controller:
    type: effort_controllers/JointPositionController
    joint: br_wheel_hinge
    pid: {p: 1.0, i: 1.0, d: 0.0}

After running the launch file, an error

load_parameters: unable to set parameters (last param was [/bl_wheel_trans_controller/pid/i=1.0]): cannot marshal None unless allow_none is enabled

What am I doing wrong? I haven't used any "none" arguments in the code but the error still exists.