Dynamic SQL Error SQL error code = -104 Token unknown

Here is the error:

Dynamic SQL Error    
SQL error code = -104    
Token unknown - line 1, column 162    
VALUE

Here is the code:

using (FbDataAdapter da = new FbDataAdapter(@"SELECT DOC_ITEMS.ID, PRODUCTS.PRODUCTKEY, PRODUCTS.NAME, DOC_ITEMS.QUANTITY, UNITS.UNIT, DOC_ITEMS.PRICE, DOC_ITEMS.TAX, (DOC_ITEMS.PRICE * DOC_ITEMS.QUANTITY) AS VALUE
                        FROM PRODUCTS INNER JOIN
                        DOC_ITEMS ON PRODUCTS.PRODUCTID = DOC_ITEMS.PRODUCTID INNER JOIN
                        UNITS ON PRODUCTS.UNITID = UNITS.UNITID
                        WHERE (DOC_ITEMS.DOCUMENTID = @DocumentID)", con))

{
    da.SelectCommand.Parameters.AddWithValue("@DocumentID", documentId);

    DataTable dt = new DataTable();
    da.Fill(dt);
    dataGridView1.DataSource = dt;

    dataGridView1.Columns["PRODUCTKEY"].AutoSizeMode = DataGridViewAutoSizeColumnMode.AllCells;
    dataGridView1.Columns["ID"].Visible = false;
    dataGridView1.Columns["NAME"].AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill;
    dataGridView1.Columns["QUANTITY"].AutoSizeMode = DataGridViewAutoSizeColumnMode.AllCells;
    dataGridView1.Columns["QUANTITY"].DefaultCellStyle.Alignment = DataGridViewContentAlignment.MiddleRight;
    dataGridView1.Columns["UNIT"].AutoSizeMode = DataGridViewAutoSizeColumnMode.AllCells;
    dataGridView1.Columns["PRICE"].AutoSizeMode = DataGridViewAutoSizeColumnMode.AllCells;
    dataGridView1.Columns["PRICE"].DefaultCellStyle.Format = "0.00 $";
    dataGridView1.Columns["TAX"].AutoSizeMode = DataGridViewAutoSizeColumnMode.AllCells;
    dataGridView1.Columns["TAX"].DefaultCellStyle.Format = "0\\%";
    dataGridView1.Columns["VALUE"].AutoSizeMode = DataGridViewAutoSizeColumnMode.AllCells;
    dataGridView1.Columns["VALUE"].DefaultCellStyle.Format = "0.00 $";
}

When I run this command through my database directly it is working ok but inside my c# code it drops this error.