Thought I'd share the solution to this annoying problem, when executing the line in the intermediate window you get the tip:
Embedded interop type 'Microsoft.Office.Interop.Excel.XlFindLookIn' is defined in both 'Excel.dll' and 'XYZ.ExcelAddin.ABC.dll'. Some operations on objects of this type are not supported while debugging. Consider casting this object to type 'dynamic' when debugging or building with the 'Embed Interop Types' property set to false.
I solved the problem by turning Embed Interop Types to False for the reference DLL Microsoft.Office.Interop.Excel