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Summary:

I have a simple CoreData-backed iOS project, with a test suite using an NSInMemoryStoreType data store.  When I perform a fetch request which uses an NSExpressionDescription (to calculate an average for a field) I get a crash with -[__NSCFNumber count]: unrecognized selector sent to instance.  When I switch the store type to NSSQLiteStoreType the fetch succeeds as expected.

Steps to Reproduce:

given an otherwise working NSFetchRequest, configure with the following code:

        [fetchRequest setResultType:NSDictionaryResultType];

        NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:@"duration"];
        NSExpression *avgExpression = [NSExpression expressionForFunction:@"average:" arguments:[NSArray arrayWithObject:keyPathExpression]];

        NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
        [expressionDescription setName:@"averageDuration"];
        [expressionDescription setExpression:avgExpression];
        [expressionDescription setExpressionResultType:NSDoubleAttributeType];

        [fetchRequest setPropertiesToFetch:[NSArray arrayWithObject:expressionDescription]];

NSArray *resultSet = [context executeFetchRequest:fetchRequest error:&error];


Expected Results:

Executing the fetch request should succeed and return a dictionary in the resultSet array, with an NSNumber for the averageDuration key


Actual Results:

An exception/crash with -[__NSCFNumber count]: unrecognized selector sent to instance. Switching to an NSSQLiteStoreType results in the request succeeding as expected.