excel find error

Hi
On
Cells.Find(What:="XYZ:", After:=ActiveCell, LookIn:= _
xlValues, LookAt:=xlPart, SearchOrder:=xlByColumns, SearchDirection:= _
xlNext, MatchCase:=False).Activate

On completion of finding this data in the sheet.
It finds all the occurrences ( in this case 20) and at the last one it pops up an error
"runtime error 91"
"Object variable or with block variable not set error."

Can you tell me how it can be fixed.
ThankYou