问为什么我不能在我的proto中BallJoint节点的‘RotationalMotor’字段中插入设备节点？

EN

      BallJoint {
        jointParameters BallJointParameters {
          anchor 0 0.05 0
        }
        device [
          RotationalMotor {
            name "motor 1"
            maxVelocity 1
          }
        ]
        device2 [
          RotationalMotor {
            name "motor 2"
            maxVelocity 1
            minPosition -1.5707963267948966
            maxPosition 1.5707963267948966
          }
        ]
        device3 [
          RotationalMotor {
            name "motor 3"
            maxVelocity 1
          }
        ]
        endPoint Solid {
          translation 0 0.05 0
          children [
            Shape {
              appearance BrushedAluminium {
              }
              geometry DEF SPHERE Sphere {
                radius 0.03
                subdivision 32
                ico FALSE
              }
            }
          ]
          boundingObject USE SPHERE
          physics Physics {
          }
        }
      }