ios - NSOutputStream doesn't write data

Question

I am downloading a file and writing its bytes to an NSOutputStream:

 self.outputFileStream = [NSOutputStream outputStreamToFileAtPath:downloadFilePath 
 append:NO];    
 [self.outputFileStream setDelegate:self];

Then, in a NSURLSession's didReceiveResponse callback:

[self.outputFileStream open];

And, once the first bytes received:

const uint8_t *bytes = (const uint8_t*)[data bytes];       
NSInteger bytesLeft = [data length];
NSInteger bytesWritten = -1;
do {
   bytesWritten = [self.outputFileStream write:bytes maxLength:bytesLeft];
   if (bytesWritten == -1) {
       break;
   }
   bytesLeft -= bytesWritten;
} while (bytesLeft > 0);

The execution stops the first time I try to write data to the stream: write:maxLength: returns -1.

To find out why this is happening, I have implemented a delegate method handleEvent: eventCode:.

For some reason, it only gets called if I place the following lines of code when initializing the stream:

[self.outputFileStream scheduleInRunLoop:[NSRunLoop mainRunLoop]
forMode:NSDefaultRunLoopMode];

and returns NSStreamEventErrorOccurred with No such file or directory description, although the directory is double-checked to be existing.