Cannot animate (0).(1) on an immutable object?

I have a ComboBox with a custom theme I wrote, and I get the error message "Cannot animate (0).(1) on an immutable object." This specifically happens when I set its selectedindex after the user selects one of the options in the combobox.

Doing some research online, I found that this is a common issue with databound items or dynamic resources. I'm not using any databound resources, but what I think is happening is since the combobox is collapsed, it tries to set the state of a button that doesn't exist. I narrowed it down to this code:

<ControlTemplate.Triggers>
  <Trigger Property="IsSelected" Value="True">
     <Setter TargetName="Border" Property="Background" Value="{DynamicResource spPressedStateBrush}" />
  </Trigger>
  <Trigger Property="IsMouseOver" Value="True">
     <Setter TargetName="Border" Property="Background" Value="{DynamicResource spOverStateBrush}" />
  </Trigger>
</ControlTemplate.Triggers>

Which depends on these Dynamic Resources:

<LinearGradientBrush x:Key="spOverStateBrush" StartPoint="0,0" EndPoint="0,1">
    <GradientStop Color="#2C8CBF" Offset="0" />
    <GradientStop Color="#2793BF" Offset="0.5" />
    <GradientStop Color="#2483BF" Offset="0.5001" />
    <GradientStop Color="#2C8CBF" Offset="1" />
</LinearGradientBrush>

<LinearGradientBrush x:Key="spPressedStateBrush" StartPoint="0,0" EndPoint="0,1">
    <GradientStop Color="#0C6C9F" Offset="0" />
    <GradientStop Color="#07739F" Offset="0.5" />
    <GradientStop Color="#04639F" Offset="0.5001" />
    <GradientStop Color="#0C6C9F" Offset="1" />
</LinearGradientBrush>

So I'm pretty sure those dynamic resources are the culprit, but how would I solve this problem?

After painstakingly trying to debug it by comparing the code to the original Control Template, I figured out that transporting all of the resources that my control uses into the file itself, and replacing all DynamicResources with StaticResources and that fixed the bug I was having.