GetDlgItemText. Write to static class member. Unresolved External.

class newtonsmethod
{
public:
	newtonsmethod();
	~newtonsmethod();
	void RunMessageLoop();
	HRESULT Initialize();
	HRESULT CreateDeviceIndependentResources();
	static LRESULT CALLBACK WindowProc(_In_  HWND hwnd, _In_  UINT uMsg, _In_  WPARAM wParam, _In_  LPARAM lParam);
	HRESULT OnRender();
	HRESULT CreateDeviceResources();
	void OnResize(UINT width,UINT height);
	void DiscardDeviceResources();
	static BOOL CALLBACK GraphItemProcedure(HWND hwndDlg, UINT message, WPARAM wParam, LPARAM lParam);
	ID2D1Factory* m_pDirect2dFactory;
	ID2D1HwndRenderTarget* m_pRenderTarget;
	ID2D1SolidColorBrush* m_pLightSlateGrayBrush;
	ID2D1SolidColorBrush* m_pCornflowerBlueBrush;
	static WCHAR szEquation[80];
	HINSTANCE m_hInstance;
	HWND m_hwnd;
	MSG msg;
};

BOOL CALLBACK newtonsmethod::GraphItemProcedure(HWND hwndDlg,
	UINT message,
	WPARAM wParam,
	LPARAM lParam)
{
	switch (message)
	{
	case WM_COMMAND:
		switch (LOWORD(wParam))
		{
		case IDOK:
			if (!GetDlgItemText(hwndDlg, IDC_EDIT1, szEquation, 80))
			{
				*szEquation = 0;
			}
				//*szEquation = 0;

			// Fall through. 
			//return TRUE;
		case IDCANCEL:
			EndDialog(hwndDlg, wParam);
			return TRUE;
		}
	}
	return FALSE;
}

When I try to build the application I get a warning and two errors:
Warning	1	warning C4715: 'WinMain' : not all control paths return a value	c:\users\eric\documents\visual studio 2013\projects\newtonsmethod\newtonsmethod\main.cpp	18	1	newtonsmethod
Error	2	error LNK2001: unresolved external symbol "public: static wchar_t * newtonsmethod::szEquation" (?szEquation@newtonsmethod@@2PA_WA)	C:\Users\Eric\documents\visual studio 2013\Projects\newtonsmethod\newtonsmethod\newtonsmethod.obj	newtonsmethod
Error	3	error LNK1120: 1 unresolved externals	C:\Users\Eric\documents\visual studio 2013\Projects\newtonsmethod\Debug\newtonsmethod.exe	newtonsmethod
I tried making szEquation a global variable declared outside of WinMain however GraphItemProcedure was unable to find that szEquation exists in that configuration
Does anyone have any suggestions on how to get this working?
Thanks!