vb.net exclude string from reflection Expression

I use relection to make a dynamic filter. But when I've a DateTime field, I want to exclude from my search date=Nothing.

 Dim Expr As Expression = Expression.Property(paramExpr, pinfo)
 Dim Typ = pinfo.PropertyType.FullName

 Dim CondPart As Expression = Nothing
 Dim toStr As Expression = Nothing
 If Typ.Contains("DateTime") Then
     toStr = Expression.Call(Expr, "ToString", Nothing, Expression.Constant("yyyyMMdd")) 'Here string to exclude "00010101" -> 01/01/0001
 Else
     toStr = Expression.Call(If(pinfo.PropertyType.IsValueType, Expr, Expression.Coalesce(Expr, Expression.Constant(String.Empty))), "ToString", Nothing)
 End If
 Dim indexOf As Expression = Expression.Call(toStr, "IndexOf", Nothing, Expression.Constant(MesDonnees.FiltreGeneral), Expression.Constant(StringComparison.OrdinalIgnoreCase))
 CondPart = Expression.NotEqual(indexOf, Expression.Constant(-1))
 If conditionGbl Is Nothing Then
     conditionGbl = CondPart
 Else
     conditionGbl = Expression.OrElse(conditionGbl, CondPart)
 End If

I try to put

toStr = Expression.Call(Expr, "ToString", Nothing, Expression.Constant("yyyyMMdd"))
toStr = Expression.NotEqual(toStr, Expression.Constant("00010101")) 

But I've error

No method 'IndexOf' exists on the type 'System.Boolean'

I Think it's between NotEqual and indexOf.

I try too

 Dim NoNullDte As Expression = Expression.Call(Expr, "ToString", Nothing, Expression.Constant("yyyyMMdd"))
 toStr = Expression.Call(NoNullDte, "IndexOf", Nothing, Expression.Constant("00010101"), Expression.Constant(StringComparison.OrdinalIgnoreCase))

But there, I've error:

No method 'IndexOf' exists on the type 'System.Int32'

Someone know How I can exclude the null date, properly?