call ToUpper in expression tree LINQ

Question

I would like to create this LINQ query:

    Result = Result.Where(Function(Row) Convert.ToString(Row(0)).ToUpper = "TEST")

I have this query already:

Result = Result.Where(Function(Row) Convert.ToString(Row(0)) = "TEST")  

with this code:

expr = Expression.Call(whereMethod, Result.AsQueryable.Expression,
       Expression.Lambda(Expression.Equal(Expression.Call(convertMethod, Expression.ArrayAccess(rowParameter, Expression.Constant(index))),
       Expression.Constant(constant)), rowParameter))

The convertMethod represents Convert.ToString, the index has the value 0, and constant has the value "TEST".

Now, I would like to add to this expression the ToUpper method.

I declared this:

convertMethod_toupper = GetType(String).GetMethod("ToUpper", New Type() {GetType(Object)})

And I found this: ToUpper in an Expression call

I should also call the ToUpper method after the Convert.ToString(Row(0)) with calling Expression.Call one more time. But how?

Thanks.

EDIT: in the meantime, I figured out, that this returns null too:

convertMethod_toupper = GetType(String).GetMethod("ToUpper", New Type() {GetType(Object)})

What is wrong with this?

If this should not be null, I think, this should be working:

expr = Expression.Call(whereMethod, Result.AsQueryable.Expression,
       Expression.Lambda(Expression.Equal(Expression.Call(Expression.Call(convertMethod, Expression.ArrayAccess(rowParameter, Expression.Constant(index))), convertMethod_toupper),
       Expression.Constant(constant)), rowParameter))

EDIT2: I got it.

convertMethod_toupper = GetType(String).GetMethod("ToUpper", System.Type.EmptyTypes)

and the above expression (in edit1) works.