Error handler can't handle mystery error

I have a search-and-replace function that worked fine until
it broke. I added an error handler so now it looks like:

Function Replace(txtMain As String, txtSearch As String, txtReplace As String)
 
On Error GoTo Trap
    Dim FoundAt As Integer
    Dim txtWork As String
    txtWork = txtMain

        Do
            FoundAt = InStr(FoundAt + 1, txtWork, txtSearch)
            txtWork = Left$(txtWork, FoundAt - 1) & txtReplace & Mid$(txtWork, FoundAt + Len(txtSearch))
            Loop While (InStr(FoundAt + 1, txtWork, txtSearch) <> 0)
    Replace = txtWork
   
Trap:
MsgBox "Replace error" & Str(Err.Number) & ": " & Err.Description, , "Error"
     
End Function

The message this generates is:
"Replace error 0:"

What is error #0 and why does it have no description?