C# - SqlDataAdapter.Fill(DataTable) Returns No Rows?

I am trying to use a SqlDataAdapter to fill a DataTable, which I use as a data source to a DataGrid. The query is fine, at least when I run it manually in SSMSE. When I do the fill operation, the table gets the right columns, but no rows. The most frustrating part of this is that the code is identical (with a different query) on a different page. I cannot post the query, but my initialization code looks like this:

SqlCommand areaDAC = new SqlCommand (areaQuery, connection);
areaDAC.Parameters.Add (new SqlParameter ("@param", System.Data.SqlDbType.NVarChar, 50));
m_areaDataAdapter = new SqlDataAdapter (areaDAC);

Then to use it:

m_areaDataAdapter.SelectCommand.Parameters["@param"].Value = "Filter Val";
DataTable table = new DataTable ();
m_areaDataAdapter.Fill (table);

At this point, table has the right number of columns and no rows. I know the parameters are being added correctly, I know that data exists for the given query.

Update (as provided by Nik in a comment):

[Code]....