Type 13 mismatch error - excel

Question

I'm receiving a type 13 mismatch error with Excel VBA. This script checks two columns and locks cells in a column once a change is made, or doesn't lock it if the user clicks the cell and clicks off without any changes. Line 5 is apparently the culprit. Any help is much appreciated.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim A As Range
Set A = Union(Range("I:I"), Range("J:J"))
If Intersect(Target, A) Is Nothing Then Exit Sub
If Target.Value = "" Then Exit Sub
ActiveSheet.Unprotect Password:="YourPassword"
Target.Locked = True
ActiveSheet.Protect Password:="YourPassword"
End Sub

Answer 1

Target is the cell or cells that have been changed. If Target is more than a single cell (e.g. pasted block of values, etc) then Target does not have a .Value. Add If Target.Count > 1 Then Exit Sub to the top of the code or loop through Target, examining each cell within Target for the .Value.
Example of the latter,
Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Union(Range("I:I"), Range("J:J"))) Is Nothing Then
On Error GoTo bm_Safe_Exit
ActiveSheet.Unprotect Password:="YourPassword"
Application.EnableEvents = True
Dim ij As Range
For Each ij In Intersect(Target, Union(Range("I:I"), Range("J:J")))
If ij.Value <> "" Then
ij.Locked = True
End If
Next ij
End If
bm_Safe_Exit:
ActiveSheet.Protect Password:="YourPassword"
Application.EnableEvents = True
End Sub
Additionally, it is not considered a 'best practise' to use the ActiveSheet property in a Worksheet_Change event macro.