Passing dataset at runtime to FastReport.Core , C#

I am using FastReport.Core 2022.1.5 nuget package in a .NET 5.0 Forms application. I have used FR Designer to generate a frx file, and I supply the data using RegisterData() at runtime. But when the Prepare is called, it throws an exception that "table is not connected to data". The data source name in frx is exactly matching the name passed to the RegisterData method.

Let me know what is missing here ?

The code flow is as below,

  FillTheDataset(); //queries using Npgsql, a postgresql db and gets back records filled in a DataSet

      Report report = new Report();

      report.Load(@"localPgReport1.frx");

      //register the dataset

      _dataSet.Tables[0].TableName = "dept";

      report.RegisterData(_dataSet,"departments");

      report.GetDataSource("departments").Enabled = true;

      //prepare the report

      report.FileName = "My Postgres Report";

      report.Prepare(); //exception in this line

FRX snippet,

<TableDataSource Name="departments" ReferenceName="departments" Enabled="true" TableName="dept">

...<DataBand Name="Data1" DataSource="departments">

.. <TableCell Name="Cell21" Text="[dept.dept_id]" Font="Arial, 10pt"/>