vba Checkbox if else condition in Subform, access - vba

Question

I have a main form with several tab controls with subforms in them. On the first tab or subform I have a "checkbox 1", which shall be:
checked if "textbox 1" is not empty
unchecked if "textbox 1" is empty
The code is directly put into the Class Object of "subform 1", that's why I thought I could use Me.
Here is my code, but I always get error messages :(
Private Sub Ctltextbox_1_AfterUpdate()
If Len(Ctltextbox_1.Value) = 0 Then
Me.checkbox_1.Value = 0
Else
Me.checkbox_1.Value = -1
End If
End Sub
That way I am getting the
Run-time error '2448': You can't assign a value to this object.
On the line that attempts to assign -1 to Me.checkbox_1.Value.

Answer 1

Try this. It is working for me. I moved it to the Ctltextbox_1_Change action. This will call the function whenever it is changed and will catch when you have nothing in the box as well. I also changed your values to True and False.
Please try this and let me know.
Private Sub Ctltextbox_1_Change()
If Len(frmSub.Ctltextbox_1.Value) = 0 Then
frmSub.CheckBox_1.Value = False
Else
frmSub.CheckBox_1.Value = True
End If
End Sub
you can also solve this in one line as mentioned below.
Private Sub Ctltextbox_1_Change()
frmSub.CheckBox_1.Value = IIf((Len(frmSub.Ctltextbox_1.Value) = 0), False, True)
End Sub

Answer 2

Ok, the simple answer is: you can not use the expression Len()= 0 for a not numeric field! So my working code for the subform is now:
Private Sub Textbox1_AfterUpdate()
If IsNull(Textbox1.Value) = False Then
Me.checkbox1.Value = True
Else
Me.checkbox1.Value = False
End If
End Sub
Thank you all for your help! :)