Cannot get this property name using this code and expression api

I have the following class and I need to get its property names:

public class PMLButtonData
{
    public int BackgroundColorID
    {
        get;
        set;
    }

    public string Callback
    {
        get;
        set;
    }
}

To get the names I'm using this function

public static string GetPropertyName<T>(Expression<Func<T, object>> lambda)
{
    MemberExpression member = lambda.Body as MemberExpression;
    PropertyInfo property = member.Member as PropertyInfo;

    return property.Name;
}

I can get the Callback property name using this code:

string name = GetPropertyName<PMLButtonData>(x => x.Callback);

But the same code for the other property doesn't work:

string name = GetPropertyName<PMLButtonData>(x => x.BackgroundColorID);

The only difference between them is the data type, so I changed the Callback to int and the code does not work with this property anymore. Why can't I get the name of a property this way if it's an integer?