User input disappearing on trying to save.

Hello everyone, I created a table with the following script.

CREATE TABLE [dbo].[GDLFAVehicleDetails](
	[CompanyId] [int] NOT NULL,
	[AssetId] [int] NOT NULL,
	[VehicleRegNo] [nvarchar](50) NOT NULL,
	/***other fields left out***/

 CONSTRAINT [PK_GDLFAVehicleDetails] PRIMARY KEY CLUSTERED 
(
	[CompanyId] ASC,
	[AssetId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY],
 CONSTRAINT [IX_GDLFAVehicleDetails_1] UNIQUE NONCLUSTERED 
(
	[VehicleRegNo] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

I needed to make VehicleRegNo to accept only unique values
so I added Iskey= true on the DAC as shown below;
However when I add the Iskey=true value the user input disappears, when I remove it user input does not disappear on the UI and user is able to save the input entered.

What could be the issue?

 

   #region VehicleRegNo
        [PXDBString(50,IsKey =true, IsUnicode = true)]
        [PXDefault]
        [PXUIField(DisplayName = "Vehicle Reg No.")]
        public virtual string VehicleRegNo { get; set; }

        public abstract class vehicleRegNo : PX.Data.BQL.BqlString.Field<vehicleRegNo>
        {
        }
        #endregion

Kindly asist
Any kind of response will be highly appreciated.