object reference not set to an instance of an object

I am trying to populate data in a repeater control but am getting a "object reference not set to an instance of an object" error. The line it points to is in red below. Any ideas on what i'm doing wrong?

string strSQL = string.Empty;
string customerIds = "1,2";
strSQL = "SELECT FirstName ,LastName, CustomerId FROM Disp_Customers WHERE CustomerID IN (" + customerIds + ")" +
"SELECT DisputeID, CustomerID FROM Disp_Disputes WHERE CustomerID IN (" + customerIds + ")";
SqlConnection cn = new SqlConnection();
cn.ConnectionString = ConfigurationSettings.AppSettings["connectionString"];
SqlDataAdapter cm = new SqlDataAdapter(strSQL, cn);
cm.TableMappings.Add("Disputes", "Disp_Disputes");
DataSet ds = new DataSet();
cm.Fill(ds, "Disp_Customers");
ds.Relations.Add("CustomerDisputes", ds.Tables["Disp_Customers"].Columns["CustomerId"], ds.Tables["Disp_Disputes"].Columns["CustomerId"]);
ds.Relations[0].Nested = true;
PrintRepeater.DataSource = ds.Tables["Disp_Customers"];
PrintRepeater.DataBind();

Check that you have the correct names for the tables and columns. You are referencing them by name and geting a null back it looks like for the table so that when you go to check for the column in that table it blows up because the table return null.