VBA Error, Subscript Out Of Range, 9

Sub Split_By_Store()

x = Cells(Rows.Count, 1).End(xlUp).Row

MsgBox x

a = "F1:F" & x

Range(a).AdvancedFilter Action:=xlFilterCopy, CopyToRange:=Range("J1"), Unique:=True

y = Cells(Rows.Count, 10).End(xlUp).Row

For b = 2 To y
Worksheets.Add.Name = Worksheets("data").Cells(b, 10)
Next b

For c = 2 To x ' copy the data now
d = Worksheets("data").Cells(c, 6)
z = Worksheets(d).Cells(Rows.Count, 1).End(xlUp).Row
Worksheets("data").Range("A" & c & ":I" & c).Copy
Worksheets(d).Cells(z + 1, 1).PasteSpecial
Next c

End Sub

This code breaks on the line z =, I get a Subscript out of range:9 error. Basically this starts with a single sheet called "data". Column F has store number and column J is created as a helper column and each unique store number is transfered there... Then sheets are created for each unique store number from the helper column. Then it's supposed to transfer the area A:I from the "data" sheet for each store that is unique from column F and transfer that to the corresponding Store number for the sheet that was created for it in the earlier step.