"Number of frames read does not equal numberof frames analyised."

Hello,
Great tool you have built, thank you very much.
I have compiled in a VM, and had it working on some small scale data, but have recently been getting this error;

"Program: chap, version 2018

Standard library runtime error (possible bug):
(exception type: St13runtime_error)
Number of frames read does not equal number of frames analyised.
"

Any ideas what this could mean?
Best,
Jonah

Some further info on this. It seems to only happen if I try to pass -out-filename into a new folder eg;
-out-filename FOLDER/data
A relatively easy workaround, but thought you may be interested nonetheless.