Silverlight - Reflection - dynamic class - Inject code in the setter - vb.net

Question

In Silverlight, I use a Dynamic class. I want to call boolean function before set property, for example :
Private _MyVar as Object
Public Property MyVar as Object
Get
return _MyVar
End Get
Set(value as Object)
If IsUpdateProp(value,"MyVar") Then _myVar = value
End Set
End Property
Private Function IsUpdateProp(value as Object, key as string) as Boolean
If value Is Nothing AndAlso GetValueProperty(key) Is Nothing
OrElse (value IsNot Nothing AndAlso
GetValueProperty(key) IsNot Nothing AndAlso
value.Equals(GetValueProperty(key))) Then
Return False
End If
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(key))
Return True`
End Function
I want to make this with reflection, my code :
Dim dp As DynamicProperty = properties(i)
Dim fb As FieldBuilder = tb.DefineField("_" & dp.Name, dp.Type,
FieldAttributes.Private)
Dim pb As PropertyBuilder = tb.DefineProperty(dp.Name,
PropertyAttributes.HasDefault, dp.Type, Nothing)
Dim isModify = GetType(Dynamic.DynamicClass).GetMethod("SetModifyField",
New Type() {GetType(Object), GetType(String)})
Dim mbSet As MethodBuilder = tb.DefineMethod("set_" & dp.Name,
MethodAttributes.Public Or
MethodAttributes.SpecialName Or
MethodAttributes.HideBySig,
Nothing, New Type() {dp.Type})
Dim genSet As ILGenerator = mbSet.GetILGenerator()
Dim mLabel = genSet.DefineLabel
genSet.Emit(OpCodes.Nop)
genSet.Emit(OpCodes.Ldarg_0)
genSet.Emit(OpCodes.Ldarg_1)
genSet.Emit(OpCodes.Box, dp.Type)
genSet.Emit(OpCodes.Ldstr, dp.Name)
genSet.Emit(OpCodes.Call, isModify)
genSet.Emit(OpCodes.Call, GetType(Convert).GetMethod("ToBoolean",
New Type() {GetType(Object)}))
genSet.Emit(OpCodes.Stloc_0)
genSet.Emit(OpCodes.Ldloc_0)
genSet.Emit(OpCodes.Brfalse_S, mLabel)
genSet.Emit(OpCodes.Ldarg_0)
genSet.Emit(OpCodes.Ldarg_1)
genSet.Emit(OpCodes.Stfld, fb)
genSet.MarkLabel(mLabel)
genSet.Emit(OpCodes.Nop)
genSet.Emit(OpCodes.Nop)
genSet.Emit(OpCodes.Ret)
At runtime, when I call setvalue on my property I get this error message:
System.Security.VerificationException: Cette opération pourrait déstabiliser le runtime." & vbCrLf & " à DynamicClass1.set_MyVar(Nullable`1 )
Thanks