C# - Extend DynamicQuery.cs To Implement A .Single Method?

I need to write some dynamic queries for a project I'm working on. I'm finding out that a significant amount of time is being spent by my program on the Count and First methods, so I started to change to .Single, only to find out that there is no such method.

The code below was my first attempt at creating one (mostly copied from the Where method), but it's not working.

public static object Single(this IQueryable source, string predicate, params object[] values)
{
if (source == null) throw new ArgumentNullException("source");
if (predicate == null) throw new ArgumentNullException("predicate");
LambdaExpression lambda = DynamicExpression.ParseLambda(source.ElementType, typeof(bool), predicate, values);
return source.Provider.CreateQuery(
Expression.Call(
typeof(Queryable), "Single",
new Type[] { source.ElementType },
source.Expression, Expression.Quote(lambda)));
}