Q:

Excel VBA "Appliction-defined or object-defined error"

I'm trying to clculate a formula that takes number from Column D and calcultes this number minus 14 in column C. 
Then, I'm triyng to autofill the range down to the first cell I calculate.
at first it was working, but now it shows me an error:

appliction-defined or object-defined error

to this line of code
Selection.AutoFill Destination:=ActiveCell.Range("A1:A" & lastrow)

If someone would help me to solve this prblem I'll be glad.
OP_wb.Sheets("Optic Main").Activate

Dim FirstRow As Range
Dim lastrow As Range

Set FirstRow = Range("C1").End(xlDown).Offset(1, 0)
Set lastrow = Range("E1").End(xlDown).Offset(0, -2)

Range("E1").End(xlDown).Offset(0, -2).Select

            Range(FirstRow, lastrow).FormulaR1C1 = "=(c4-14)"
            Selection.AutoFill Destination:=ActiveCell.Range("A1:A" & lastrow)

A:

Instead of 
Selection.AutoFill Destination:=ActiveCell.Range("A1:A" & lastrow)

try
Selection.AutoFill Destination:=ActiveCell.Range("A1:A" & lastrow.Row)

