Forms Data Controls :: How To Disable Button Inside EditItemTemplate Inside DetailsView

I have a boolean / bit value in the database called "is_paid". Depending on this value I want to Disable a button. (the same would go for example to set a textbox not Visible, but anyway)

What I want is to Disable the button, if my boolean is 1 (true).

First I tried to convert my boolean value to text:

<asp:Button
ID="btnCalculatePayment"
runat="server"
Text="Submit"
onclick="btnCalculatePayment_Click" Enabled='<%#
Convert.ToBoolean(Eval("is_paid")) ? "False" : "True" %>'
/>

But got an error:

CS0030: Cannot convert type 'string' to 'bool'
However, if I use it one a Label it works fine: <asp:Label
ID="lblIsPaid"
runat="server"
Text='<%#
Convert.ToBoolean(Eval("is_paid")) ? "True" : "False" %>'></asp:Label>

So another trial I did was to CAST the data from the database to be the text that I want, like so:

SELECT not_paid = CASE WHEN is_paid = 0 THEN 'True' WHEN is_paid = 1 THEN 'False' END ....

And my code changed to:

<asp:Button
ID="btnCalculatePayment"
runat="server"
Text="Submit"
onclick="btnCalculatePayment_Click" Enabled='<%#
Bind("not_paid") %>'
/>

But got this error:

System.InvalidCastException was unhandled by user code
Message=Specified cast is not valid.
Source=App_Web_absjd3ob
StackTrace:
at ASP.manager_aspx.__DataBinding__control74(Object sender, EventArgs e) in c:Documents and SettingsAdminMy DocumentsVisual Studio 2010ProjectsWebManager.aspx:line 219
at System.Web.UI.Control.OnDataBinding(EventArgs e)
at System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding)
at System.Web.UI.Control.DataBind()
at System.Web.UI.Control.DataBindChildren()

So I wonder, is it possible to change the "Enabled" property based on by boolean value?