How to solve the following error: GenericArguments[0], 'AddFilterOnProjectTaskWeekly.ProjTaskAttribute+CONTAINS_LABOUR', on 'PX.Data.Current`1[Field]' violates the constraint of type parameter 'Field'.

public class ProjTaskAttribute : ProjectTaskAttribute
    {
        public const string Labor = "LABOR";
        public ProjTaskAttribute(Type projectID) : base(projectID)
        {

            Type Labor = typeof(CONTAINS_LABOUR);
            Type SearchType =
                BqlCommand.Compose(
                typeof(Search<,>),
                typeof(PMTask.taskID),
                typeof(Where<,,>),typeof(PMTask.projectID),typeof(Equal<>),typeof(Current<>),projectID,
                typeof(And<,>),typeof(PMTask.description),typeof(Like<>),typeof(Current<>), Labor
                );

            PXDimensionSelectorAttribute select = new PXDimensionSelectorAttribute(DimensionName, SearchType, typeof(PMTask.taskCD),
                         typeof(PMTask.taskCD), typeof(PMTask.description), typeof(PMTask.status));
            select.DescriptionField = typeof(PMTask.description);
            select.ValidComboRequired = true;

            _Attributes.Add(select);
            _SelAttrIndex = _Attributes.Count - 1;

            Filterable = true;
        }

        public sealed class CONTAINS_LABOUR : BqlType<IBqlString, string>.Constant<CONTAINS_LABOUR>
        {
            /// <exclude/>
            public CONTAINS_LABOUR() : base("LABOR") { }
        }
    }

How to solve the following error: GenericArguments[0], 'AddFilterOnProjectTaskWeekly.ProjTaskAttribute+CONTAINS_LABOUR', on 'PX.Data.Current`1[Field]' violates the constraint of type parameter 'Field'.