DataGridTextColumn binding issue

Question

I am having trouble binding to IsReadOnly on aDataGridTextColumn.

                                    <DataGridTextColumn Binding="{Binding Path=OffsetRelative}" IsReadOnly="{Binding IsReadOnly}">
                                    <DataGridTextColumn.HeaderTemplate>
                                        <DataTemplate>
                                            <CheckBox Content="Relative Offset" IsChecked="{Binding ElementName=ViewReportFileUserControl, Path=DataContext.IsSelectedTcfOffsetRelative}" Margin="0,0,10,0"/>
                                        </DataTemplate>
                                    </DataGridTextColumn.HeaderTemplate>
                                </DataGridTextColumn>

Both OffsetRelative and IsReadOnly are public properties in the same class. The Binding to OffsetRelative works fine but the Binding to IsReadOnly generates the following run time output:

System.Windows.Data Error: 2 : Cannot find governing FrameworkElement or FrameworkContentElement for target element. BindingExpression:Path=IsReadOnly; DataItem=null; target element is 'DataGridTextColumn' (HashCode=2149042); target property is 'IsReadOnly' (type 'Boolean')

Why would one binding work and not the other?