A spreader (24) comprises a main frame carrying container connector arrangements configured to engage with a transport container (10); a rotator enabling rotation of the main frame in relation to a crane bracket about a substantially vertical rotation axis (A2); a rotation motor configured to, responsive to a rotation control signal, operate the rotator to rotate the main frame; a rotation detector configured to detect rotation of the main frame in relation to the crane bracket; and a control system configured to, based on a discrepancy between the rotation control signal and a rotation detected by the rotation detector, generate a rotation alert signal.