Macro For Next & Previous Sheet Selection

I have created a toolbar that has "Previous Worksheet" and "Next Worksheet" buttons on it with the following macros assigned:

Sub NextSheet()
On Error Resume Next
If ActiveSheet. Name = "Problem Listing" Then
MsgBox "This is the last worksheet."
Else
ActiveSheet.Next.Select
End If
End Sub

Sub PreviousSheet()
On Error Resume Next
If ActiveSheet.Index = 1 Then
MsgBox "This is the first worksheet."
Else
ActiveSheet.Previous.Select
End If
End Sub

This seems really simple but the code crashes after 4 or 5 repeated clicks of the button.