Serializing LoggingEvent with null entries in Properties throws exception

public virtual void GetObjectData(SerializationInfo info, StreamingContext context)
{
 foreach (DictionaryEntry dictionaryEntry in this.InnerHashtable.Clone() as IDictionary)
 {
   string key = dictionaryEntry.Key as string;
   object obj = dictionaryEntry.Value;  // When dictionaryEntry.Value is null
 bool isSerializable = obj.GetType().IsSerializable; // NullReferenceException
 if (key != null && obj != null && isSerializable)
 info.AddValue(XmlConvert.EncodeLocalName(key), obj);
 }
 }

public virtual void GetObjectData(SerializationInfo info, StreamingContext context)
{
foreach (DictionaryEntry dictionaryEntry in this.InnerHashtable.Clone() as IDictionary)
{
string key = dictionaryEntry.Key as string;
object obj = dictionaryEntry.Value;
bool isSerializable = obj == null ? false :  obj.GetType().IsSerializable;
if (key != null && obj != null && isSerializable)
info.AddValue(XmlConvert.EncodeLocalName(key), obj);
}
}