WORLDBROWSER_API undefined class

I’m attempting to modify WorldBrowserModule.h to include some extra customization hooks, similar to the following:

class WORLDBROWSER_API FCustomizationHooks
{
public:
	DECLARE_DELEGATE(FSomeDelegate);
	static FSomeDelegate OnEvent;
};

The OnEvent is then defined in the corresponding WorldBrowserModule.cpp:

FCustomizationHooks::FSomeDelegate FCustomizationHooks::OnEvent;

However, when building I get the following error:

1>D:\RJ\UnrealEngine\Engine\Source\Editor/WorldBrowser/Public/WorldBrowserModule.h(84): error C2079: ‘FCustomizationHooks’ uses undefined class ‘WORLDBROWSER_API’

Of course, if I remove the WORLDBROWSER_API, the WorldBrowser module itself builds, but then I get (expected) linker errors attempting to actually bind to the delegate due to undefined external symbols.

I’ve already checked out 'xxx' uses undefined class 'WORLDBROWSER_API' error - Plugins - Epic Developer Community Forums, however it is not relevant.

Why is WORLDBROWSER_API undefined in this context? What is the correct method to expose public functionality outside of adding virtual methods to the FWorldBrowserModule interface to the WorldBrowser module?