Serialize null reference object

I'm trying to 'serialize' a certain type by iterating through its properties and write down every value type into a dictionary.

Pseudo

serialize(object)
    foreach(prop in object.GetType().GetProperties()

        value = prop.GetValue(object);
        if (prop.PropertyType.isValueType)
            writeToDict(prop.Name, value)
        else
            serialize(value)

given a certain type

public class ComplexType {
    public string Prop1 { get; set; }
    public int Prop2 { get; set; }
    public OtherType Prop3 { get; set; }
}

public class OtherType {
    public string Prop4 { get; set; }
}

Now, when I create an instance of ComplexType and try to 'serialize' it, it works fine until it iterates over Prop3 (OtherType). When I try to call value.GetType() on it, I run into a NullReferenceException because, of course, there has never been a reference set to Prop3.

Is there any way to work around this? How do other serialization frameworks do this? (I mean, I can't even create a default instance of that type because I don't know the type at runtime!)

Oh, and yes, I can't skip the property because I'm interested in the structure (even if it doesn't have a value set).