Enumeration-based property with initialiser fails to load in tool pane

I encountered what is likely an obscure issue today while using an existing enumeration to populate a drop down list in the tool pane of a SharePoint web part.

The web part didn't require a custom toolpart so I'm referring to a web part property defined within the web part class itself:

[SPWebCategoryName ("TWA")]
[FriendlyNameAttribute ("Region Override")]
[WebDescription ("Explicitly configure the region displayed when the mode is set to Region.")]
[Personalizable (PersonalizationScope.Shared, false)]
[WebBrowsable (true)]
public WARegions RegionOverride { get; set; }

Notice how the public RegionOverride property has a type of WARegions. Normally, the end result is an additional custom property displayed in the web part tool pane; in this case, the WARegions type is an enumeration and SharePoint therefore renders this as a drop down list:

I say normally because today I was only getting a disabled DDL containing a single item ("Perth"). SharePoint was also kind enough to present me with this vaguely useful error message:

Some of the properties for this Web Part cannot be displayed properly. For more information, see your site administrator.

The WARegions enumeration is an existing enum I chose to reuse for the sake of convenience. Upon further inspection, I noticed the first element in the enum (Perth) was explicitly assigned a value of 1. Acting on my suspicion, I removed the explicit assignment and the DDL would then display as expected.

As a final test to confirm whether the explicit initialisation was somehow behind the single list item being displayed, I set default values for all elements in the enum but the error returned. Fortunately in my case I can do away with the explicit initialisation (which I'm not keen on—I prefer to initialise enums myself).

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