Code "debug" error msg

The two code extracts below are associated with two separate worksheets in the same workbook. They work fine. They simply are used to assign either a PO# or a Temp Unit #, located in a third sheet, in their respective sheets.

The problem is that when I'm in either the Master or the Work Orders sheets where these two codes are used, and I do something simple like drag down some values or paste something in a cell, I get the "Run time error "13" type mismatch error" in the popup debug window for my VBA code. And in each case, it highlights the codeline I've color coded below:

Work Orders Code:

Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
'    If Target.Count > 1 Then Exit Sub
    If Not Intersect(Target, Range("K3:K1500")) Is Nothing And UCase(Target.Value) = "X" Then
        Range("L" & Target.Row).Value = Sheets("Assign").Range("E" & Rows.Count).End(xlUp).Offset(1, -1).Value
        Sheets("Assign").Range("E" & Rows.Count).End(xlUp).Offset(1, 0).Value = "X"
       
    End If
End Sub

Master Code

Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
'    If Target.Count > 1 Then Exit Sub
    If Not Intersect(Target, Range("D3:D1000")) Is Nothing And UCase(Target.Value) = "X" Then
        Range("C" & Target.Row).Value = Sheets("Assign").Range("B" & Rows.Count).End(xlUp).Offset(1, -1).Value
        Sheets("Assign").Range("B" & Rows.Count).End(xlUp).Offset(1, 0).Value = "X"
   
    End If
End Sub

My problem is I'm so new to this type of coding that I'm totally lost as to what it is really telling me or what to do. It doesn't keep me from doing what I need to do nor does it interfere with what the code does..I just don't know how to fix it.

I've attached the sample spreadsheet.