ASObjC Runner error: connection is invalid

I am using ASObjC Runner for its XML parsing capabilities. I'm getting a strange result when trying to use the 'extract from XML' command: ASObjC Runner got an error: Connection is invalid.

The command I was using was: set oneValue_asList to extract from XML someXML matching nodeXPath expression {oneKeyName} without including output element where someXML was a valid block of XML, nodeXPath was an XPath to the desired node, and oneKeyName was the item whose value I wanted to read. I do this for many different key names and many different XPaths. It is hard to debug because it seems to get this error at different points every time I run the whole script. I haven't discovered a particular key name or XPath that causes the error. One that fails once might succeed the next time, without any change to the underlying XML or key name and XPath choices.

I don't see anything online that would explain this. I am running this in Yosemite, 10.10.1, and have the most recent version of AsObjC Runner. Any ideas on what might be causing this? The XML being processed is fairly large. I've even tried quitting AsObjC Runner periodically during the script to see if it is leaking memory. That seems to have no effect.