Why was I getting a "java.lang.IllegalArgumentException: Type mismatch" when trying a projection query on GAE

I was about to ask the following Q on StackOverflow.com but ended up finding the answer myself before even posting so I decided to make this into a new blog post.

I have two code snippets which I am trying to run on the development server of Google App Engine SDK v1.8.8 for Java.

The first:

    return datastore.get(KeyFactory.createKey(Kinds.Provider,provider_id));
    
works as expected and returns the Provider Entity from the Data Store.

But when I tried to change it to a projection Query so that it will return only some of the fields like this: 

    Filter filter=new FilterPredicate(Entity.KEY_RESERVED_PROPERTY,
                                      Query.FilterOperator.EQUAL,
                                      KeyFactory.createKey(Kinds.Provider,provider_id));
    Query q = new Query(Kinds.Provider)
                .setFilter(filter)
.addProjection(new PropertyProjection("address", String.class))
       .addProjection(new PropertyProjection("last_modified", String.class));;
}
    PreparedQuery pq = datastore.prepare(q);
    log.info("query:" + q.toString());
    
    Entity result = pq.asSingleEntity();

I am getting a the following Exception:


    java.lang.IllegalArgumentException: Type mismatch.
    at com.google.appengine.repackaged.com.google.common.base.Preconditions.checkArgument(Preconditions.java:96)
    at com.google.appengine.api.datastore.RawValue.asType(RawValue.java:61)
    at com.google.appengine.api.datastore.PropertyProjection.getValue(PropertyProjection.java:65)
    at com.google.appengine.api.datastore.EntityTranslator.createFromPb(EntityTranslator.java:29)
    at com.google.appengine.api.datastore.QueryResultsSourceImpl.processQueryResult(QueryResultsSourceImpl.java:199)
    at com.google.appengine.api.datastore.QueryResultsSourceImpl.loadMoreEntities(QueryResultsSourceImpl.java:106)
    at com.google.appengine.api.datastore.QueryResultIteratorImpl.ensureLoaded(QueryResultIteratorImpl.java:155)
    at com.google.appengine.api.datastore.QueryResultIteratorImpl.nextList(QueryResultIteratorImpl.java:110)
    at com.google.appengine.api.datastore.LazyList.forceResolveToIndex(LazyList.java:93)
    at com.google.appengine.api.datastore.LazyList.resolveToIndex(LazyList.java:73)
    at com.google.appengine.api.datastore.LazyList.resolveToIndex(LazyList.java:56)
    at com.google.appengine.api.datastore.LazyList.isEmpty(LazyList.java:260)
    at com.google.appengine.api.datastore.PreparedQueryImpl.asSingleEntity(PreparedQueryImpl.java:74)

as I was copying This exception For my StackOverflow.com Q I discovered that there was another exception before that

12:09:18 PM com.google.appengine.api.datastore.dev.LocalCompositeIndexManager updateIndexFile
SEVERE: Unable to write some_path\war\WEB-INF\datastore-indexes.xml
java.io.FileNotFoundException: some_path\war\WEB-INF\appengine-generated\datastore-indexes-auto.xml (Access is denied)

Now it makes more sense...
Projection Queries require indexes to be created as explained here If the datastore-indexes-auto.xml is read only... well you now know what happens then...

well...
I thought this was the end of it but no... After making the file writeable, GAE indeed added an index to it but I still get the original IllegalArgumentException 

logging the querys .toString() yeilded:

SELECT last_modified, address FROM Provider WHERE __key__ = Provider(99)


Finally It turned out the error was absolutely justified...
In:
.addProjection(new PropertyProjection("last_modified", String.class));;

Last Modified was a Long and not a String...