Property SetValue returns TargetParameterCountExce

TL;DR

I have the class Type for a XmlSerializer which throws a TargetParameterCountException on prop.SetValue().

Class `Type`

The class is called from my List Types

[System.Xml.Serialization.XmlArrayItemAttribute("Type", IsNullable = false)]
public List<Type> Types { get; set; }

and the class itself

[System.SerializableAttribute()]
[System.ComponentModel.DesignerCategoryAttribute("code")]
[System.Xml.Serialization.XmlTypeAttribute(AnonymousType = true)]
public partial class Type
{
    [System.Xml.Serialization.XmlAttributeAttribute()]
    public int TypeCode { get; set; }

    [System.Xml.Serialization.XmlAttributeAttribute()]
    public int NumberOfUnits { get; set; }
    public bool ShouldSerializeNumberOfUnits() { return NumberOfUnits > 0; }
}

My Property Set Funktion

From: Xerillio answer for "Insert values in class without explicit writing them"

It's meant to automatically generate the full class with all subclasses filled with demo data.

public static void SetDefaults(object testObj)
{
    PropertyInfo[] propertyInfos = testObj.GetType().GetProperties();
    foreach (PropertyInfo propertyInfo in propertyInfos)
    {
        if (propertyInfo.Name == "Count" || propertyInfo.Name == "Capacity" || propertyInfo.SetMethod == null)
        {
            continue;
        }

        var propType = propertyInfo.PropertyType;
        if (propType == typeof(int))
        {
            // [...]
        }
        else
        {
            var ctor = propType.GetConstructor(Type.EmptyTypes);
            var propertyObject = ctor.Invoke(new object[0]);
            SetDefaults(propertyObject);
            propertyInfo.SetValue(testObj, propertyObject);
        }
    }
}

The Problem

Now if the propertyObject is written and ready to be set in propertyInfo it through the TargetParameterCountException.
May it has something to do with the unset index and count properties, but I can't find anything by STFW.

Do I need to set the count and index properties anywhere or does it have a problem with the ShouldSerialize... Variable?