Simulink/ROS2 error when selecting message type:

When I try to publish or subscribe with a custom message type, I get the following error:

Error evaluating 'MaskDialog' callback of ROS2 Publish block (mask) 'untitled/Publish'. Callback string is 'ros.slros2.internal.block.PublishBlockMask.dispatch('messageTypeSelect', gcb, 'ros.slros2.internal.dlg.MessageTypeSelector');'

Expected input number 2, MessageList, to be nonempty.

Any help would be useful and greatly appreciated.