Null Response From Instruments

When looking at the Appium logs, after Appium attempts to find an element and retrieve the XML from instruments, instruments responds with the following:


My question is, what causes a response like this to occur? Is this an issue with Appium being able to communicate with instruments or an issue with instruments obtaining the device hierarchy?

I am running against Appium v1.4.13, IOS 9.1, Java Client, Yosemite v10.10.5