Generic extension method to see if an enum contains a flag

I based this method off of a bunch of SO & Google searches, and a by using reflector to see what MS did for the .NET 4 HasFlags method.

public static class EnumExt
{
    /// <summary>
    /// Check to see if a flags enumeration has a specific flag set.
    /// </summary>
    /// <param name="variable">Flags enumeration to check</param>
    /// <param name="value">Flag to check for</param>
    /// <returns></returns>
    public static bool HasFlag(this Enum variable, Enum value)
    {
        if (variable == null)
            return false;

        if (value == null)
            throw new ArgumentNullException("value");

        // Not as good as the .NET 4 version of this function, but should be good enough
        if (!Enum.IsDefined(variable.GetType(), value))
        {
            throw new ArgumentException(string.Format(
                "Enumeration type mismatch.  The flag is of type '{0}', was expecting '{1}'.",
                value.GetType(), variable.GetType()));
        }

        ulong num = Convert.ToUInt64(value);
        return ((Convert.ToUInt64(variable) & num) == num);

    }

}

Notes:

• This handles nulls
• Does type checking
• Converts to a ulong, and can handle any positive enum value. Microsoft cautions against the use of negative flags enumerations anyway:
  

  Use caution if you define a negative number as a flag enumerated constant because many flag positions might be set to 1, which might make your code confusing and encourage coding errors.