Passing OpenArg To Listbox On Other Form

On the first form I have a button with the following OnClick event :

Private Sub Rispondi_Click()
acbReceiveMail
On Error GoTo Err_Rispondi_Click
Dim stDocName As String
Dim stLinkCriteria As String
stDocName = "frmSendMail"

[Code] ....

On the form opened upon click ("frmSendMail") I am trying to populate the "cboTo" combobox with the "txtFrom" value from first form. In the OnLoad event I have written :

Private Sub Form_Load()
Me.cboTo = Me.OpenArgs
End Sub

Anyway after clicking the button that should open "frmSendMail" I get a popup saying : "An expression you entered is the wrong data type for one of the arguments" (run-time error '2498') ....