Trying to set objective cell of opensolver on VBA

Hello.

I read the opensolver API and I'm trying to set the objective function and the decision variables.
I'm using an indirect refference to the range. It is written at sheets(2), cells B3 and B4.

Cell B3 = $C$89
Cell B4 = $C$92:$C$94,$C$113:$C$115,$C$134:$C$136,$C$155:$C$157

Then I used this code:

Dim stringobj As String
Dim stringvar As String
        
stringobj = Sheets(2).Range("B3")
stringvar = Sheets(2).Range("B4")
        
setobjectivefunctioncell (Range(stringobj))
SetDecisionVariables (Range(stringvar))

This returns the error 424, "object is mandatory".
What am I doing wrong?

Thanks for the attention!