why RDLC Report Show #ERROR when printing? - vb.net

Question

i have a problem that has been bothering me for more than three weeks. i have searched for the solution in several forums, but without success. i am creating a small application with windows form using .net framwork 4.7.2. and RDLC report.
my problem is the following:
I have created an RDLC report and it takes a costomized code that works apparently well. but when I launch the print I have this message #ERROR. for that to come back to normal, I have to refresh and it's disturbing.
please help me with any suggestion.
Good File
as you can see in the picture above, the report is working
when i export or preview, this is what happens as shown in the image below
Bad File
here is the costomized code that I use in my report code.
<pre>Function SumLookup(ByVal items As Object()) As Decimal
If items Is Nothing Then
Return Nothing
End If
Dim suma As Decimal = New Decimal()
Dim ct as Integer = New Integer()
suma = 0
ct = 0
For Each item As Object In items
suma += Convert.ToDecimal(item)
ct += 1
Next
If (ct = 0) Then return 0 else return suma
End Function
</pre>
the expression of my textbox
=Round(cdbl(Code.SumLookup(LookupSet(Fields!Matiere.value, Fields!Matiere.value, Fields!NoteEleve.Value*Fields!Coef.Value, "Bulletin"))),2)
please help me. I don't know what to do.
I tried this function in the code area of the report
<pre>Function SumLookup(ByVal items As Object()) As Decimal
If items Is Nothing Then
Return Nothing
End If
Dim suma As Decimal = New Decimal()
Dim ct as Integer = New Integer()
suma = 0
ct = 0
For Each item As Object In items
suma += Convert.ToDecimal(item)
ct += 1
Next
If (ct = 0) Then return 0 else return suma
End Function
</pre>
and this one in the textbox
=Round(cdbl(Code.SumLookup(LookupSet(Fields!Matiere.value, Fields!Matiere.value, Fields!NoteEleve.Value*Fields!Coef.Value, "Bulletin"))),2)