Slate compilation issue on OS X

I need to know whether a SpinBox is in a text editing state for a virtual keyboard implementation. The only way I’ve found is to inherit from SSpinBox and provide a function that returns this information (the existing function is protected). This works fine on Windows, but I can’t get my very simple Slate widget compiled on OS X. This is the whole code:

#pragma once

#include "SSpinBox.h"

template<typename NumericType>
class SEnhancedSpinBox
	: public SSpinBox<NumericType>
{
public:
	bool IsKeyboardEditing()
	{
		return IsInTextMode();
	}
};

Compilation fails with the message: “SEnhancedSpinbox.h:16:10: Use of undeclared identifier ‘IsInTextMode’”. If I try to copy the code from SSpinBox::IsInTextMode() instead of calling it, I get the same problem with undeclared members of SSpinBox. Again, all of this works fine in Windows, what could be the problem?