Cannot Resolve Symbol

I am receiving the warning Cannot Resolve Symbol with the folowing code block in version 3.0.2. Any ideas?

public Nullable]]> Number
{
get { return _Number; }
set
{
if (value <= 0)
throw new ArgumentOutOfRangeException("Number", value, "Till Number must be greater then 0");
_Number = value;
}
}

The ArgumentOutOfRange parameters are as follows:
(String paramName, Object actualValue, String message)