Object Variable or with block not set

I am having an issue setting the value of a cell with some variables. I am getting the error in the post title, but I cannot find where this error is coming from.

Dim FirstFourRunNo As String, LastTwoRunNo As String, GrowthPrefix As String
Dim wsWafer As Worksheet
Dim RunNo As String
RunNo = Mid(ThisWorkbook.Name, 17, 6)
GrowthPrefix = InputBox("Please enter the wafer prefix (SPV, SPW, or SPL)", "Wafer Prefix")
FirstFourRunNo = Mid(RunNo, 1, 4)
LastTwoRunNo = Mid(RunNo, 5, 2)
Range("W27:Y27").Value = GrowthPrefix & FirstFourRunNo & "-" & LastTwoRunNo & wsWafer & "Fine X1"

Am I using too many variables for the .value part? Is this method not accepted in VBA?
Any ideas?

It's because you haven't set wsWafer to anything it is a worksheet object = Nothing at the moment. I assume you want to print the name, set the worksheet first then use wsWafer.Name

I forgot to include that part about wsWafer. There are 700+ lines in the macro and I only wanted to include the part that was relevant to the error.

Dim FirstFourRunNo As String, LastTwoRunNo As String, GrowthPrefix As String
Dim wsWafer As Worksheet
Dim RunNo As String
RunNo = Mid(ThisWorkbook.Name, 17, 6)
GrowthPrefix = InputBox("Please enter the wafer prefix (SPV, SPW, or SPL)", "Wafer Prefix")
FirstFourRunNo = Mid(RunNo, 1, 4)
LastTwoRunNo = Mid(RunNo, 5, 2)

For Each wsWafer In Sheets(Array("A-G", "A-N", "B-G", "B-N", "C-G", "C-N", "D-G", "D-N", "E-G", "E-N", "F-G", "F-N", _
                                    "G-G", "G-N", "H-G", "H-N", "I-G", "I-N", "J-G", "J-N"))

Range("W27:Y27").Value = GrowthPrefix & FirstFourRunNo & "-" & LastTwoRunNo & wsWafer & "Fine X1"

Next wsWafer

That is with the lines declaring wsWafer. I just tried wsWafer.Name and it worked!. Thanks NigelFrost