A formula in a cell could not be converted

I have started getting a message when I try to save a workbook after having amended a column in a working Workbook. Message is:-
A formula in a cell (Name:[workbookname.xls]worksheetname!Totals) could not be converted because it contains a function that is not available in the file format to which you are saving.

This occurs for each cell I have amended in a particular column.
The original formula was:-
=IF(ISERROR('Structure 2'!CnD);"";'Structure 2'!CnD)
which worked fine.

Amended formula is:-
=IF(ISERROR('Structure 2'!CnD);"";HYPERLINK("http://www.mywebspace.co.uk/dartford/card 1.mht";'Structure 2'!CnD))
CnD in the Structure worksheet is a valid Name for a column.

Any assistance would be appreciated.