I keep getting the 'out of range' error and I'm stumped.

Here is my code:
Workbooks("Performance-Oct-09.xlsm").Worksheets("SerU").Activate

'Copies range of master formula cells to appropriate range in worksheet to
match row of "End_Benchmark" date
With Sheets("SerU")
.Range("c1:h1").Copy _
Destination:=.Range("c" & .Range("b1"))

- the program stops at the "With Sheets("SerU") command, inferring that
there is no worksheet named "SerU" -- BUT THERE DEFINITELY IS.... I'm
stumped...
I'm trying to copy some cells from row 1 to a row further down and this code
works elsewhere in the macro. Any ideas??? tks