Extension methods must be defined in a non-generic static class

I’m getting the error:

Extension methods must be defined in a non-generic static class

On the line:

public class LinqHelper

Here is the helper class, based on Mark Gavells code. I’m really confused as to what this error means as I am sure it was working fine when I left it on Friday!

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Linq.Expressions;
using System.Reflection;

/// <summary>
/// Helper methods for link
/// </summary>
public class LinqHelper
{
    public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, string property)
    {
        return ApplyOrder<T>(source, property, "OrderBy");
    }
    public static IOrderedQueryable<T> OrderByDescending<T>(this IQueryable<T> source, string property)
    {
        return ApplyOrder<T>(source, property, "OrderByDescending");
    }
    public static IOrderedQueryable<T> ThenBy<T>(this IOrderedQueryable<T> source, string property)
    {
        return ApplyOrder<T>(source, property, "ThenBy");
    }
    public static IOrderedQueryable<T> ThenByDescending<T>(this IOrderedQueryable<T> source, string property)
    {
        return ApplyOrder<T>(source, property, "ThenByDescending");
    }
    static IOrderedQueryable<T> ApplyOrder<T>(IQueryable<T> source, string property, string methodName)
    {
        string[] props = property.Split('.');
        Type type = typeof(T);
        ParameterExpression arg = Expression.Parameter(type, "x");
        Expression expr = arg;
        foreach (string prop in props)
        {
            // use reflection (not ComponentModel) to mirror LINQ
            PropertyInfo pi = type.GetProperty(prop);
            expr = Expression.Property(expr, pi);
            type = pi.PropertyType;
        }
        Type delegateType = typeof(Func<,>).MakeGenericType(typeof(T), type);
        LambdaExpression lambda = Expression.Lambda(delegateType, expr, arg);

        object result = typeof(Queryable).GetMethods().Single(
                method => method.Name == methodName
                        && method.IsGenericMethodDefinition
                        && method.GetGenericArguments().Length == 2
                        && method.GetParameters().Length == 2)
                .MakeGenericMethod(typeof(T), type)
                .Invoke(null, new object[] { source, lambda });
        return (IOrderedQueryable<T>)result;
    }
}