Finding a range from different columns and inserting a value - vba

Question

My current challenge is that I have a list of data that has been pulled in from another workbook in Columns A-K which all finish on the same row. I want to label the data with all the same value in Column L
The code I have is trying to find the last cell in Column A and the last cell in Column L.
I am then trying to use the range between these two rows to insert a value across the range in Column L:
Sub FillTestType(TestType As String)
Dim rng As Range
Dim wsh As Worksheet
Dim Row As Long
Dim EndCell As Long
Dim TopCell As Long
Set wsh = Worksheets("Sheet1")
wsh.Activate
EndCell = Cells(Rows.count, "A").End(x1Up).Row
TopCell = Cells(Rows.count, "L").End(x1Up).Row
rng = Range(Cells(TopCell, 12), Cells(EndCell, 12)).Value
rng = TestType
End Sub
I keep getting a runtime error - Any help would be greatly appreciated! Or if I am being stupid and there is a better way to tackle the problem please do let me know.

Answer 1

Something like this should work. Mainly collating the comments:
xlUp rather than x1Up (and Option Explicit to catch that yourself in future!)
+1 to TopCell so that you don't overwrite your last value in that row
Fully qualified your Range and Cell references using With and .
Cleaned up variables which aren't necessary -- e.g. for multiple future use or readability
Option Explicit
Sub FillTestType(TestType As String)
Dim EndCell As Long
Dim TopCell As Long
With Worksheets("Sheet1")
EndCell = .Cells(.Rows.Count, "A").End(xlUp).Row
TopCell = .Cells(.Rows.Count, "L").End(xlUp).Row + 1
.Range(.Cells(TopCell, 12), .Cells(EndCell, 12)).Value = TestType
End With
End Sub