Cannot find IsChecked property in WPF Checkbox

I have a WPF app with a few checkboxes. I am able to set the IsChecked property in XAML, however, when I go to set it programatically, there is no such property. From everything I read, this property (IsChecked) is how I should be setting the value.

The XAML is below:

<StackPanel Orientation="Horizontal">
  <Label FontWeight="Bold">Alarm 1</Label>
  <CheckBox Name="ckbAlarm1" VerticalAlignment="Center"></CheckBox>
</StackPanel>

if (Regex.Matches(alarm1Data.ToString(), "ALARM 1 DISABLED").Count == 1)
{
   spAlarm1.IsEnabled = false;
   // ckbAlarm1.IsChecked = false; // <--- property not found
}