Error uploading artifact to backend

I have been creating deliberately bad artifacts to test automatic rollback.

dd if=/dev/zero of=image.dat bs=1M count=0 seek=16
ARTIFACT_TOOL=/work/dmoseley/Mender/yocto/morty/build/beaglebone/tmp/sysroots/x86_64-linux/usr/bin/mender-artifact
${ARTIFACT_TOOL} write rootfs-image -t beaglebone -n mender-image-1.1 -u image.dat -o core-image-full-cmdline-beaglebone.mender
${ARTIFACT_TOOL} write rootfs-image -t vexpress-qemu -n mender-image-1.1 -u image.dat -o core-image-full-cmdline-vexpress-qemu.mender
${ARTIFACT_TOOL} write rootfs-image -t raspberrypi2 -n mender-image-1.1 -u image.dat -o core-image-full-cmdline-raspberrypi2.mender

This is based on the automated test script for this feature. When I try to load the artifact into my backend, I get a variety of errors:

• Artifact couldn't be uploaded. No size provided before the artifact part of the message or the size value is wrong.
• Artifact couldn't be uploaded. There was an error uploading the artifact
• Artifact couldn't be uploaded. internal error

Typically this will eventually succeed if I retry the upload but sometimes it takes 4-5 attempts.

I verified this behavior with the 1.0.x, master and git versions of the canned integration environment. I primarily used the mender-artifact tool that was built with my Yocto builds but also verified with the latest master branch version.