Where is this FileNotFoundException coming from when building asset bundles?

I'm building a few hundred asset bundles in a loop. Unity builds the first one fine, but on all subsequent bundles I get the following error in the Editor log:

Responsefile: Temp/UnityTempFile-b94bc68e704bb431c8e352f6fb9dfbce Contents: Could not fetch error messages from compilerFileNotFoundException: Could not find file "/Users/[myusername]/dev/Unity/[myprojectname]/Temp/UnityTempFile-b94bc68e704bb431c8e352f6fb9dfbce".

This exception is thrown from within my call to BuildPipeline.BuildPlayer. I use the same bundle building code in another project and it runs perfectly. I've tried making a new project and importing all assets, but I still get this error.

This is driving me crazy. Any ideas?

EDIT: If a bundle exists, the script doesn't overwrite it. When I run the script manually, I am able build the next bundle. This is not a viable solution, though, because I have over 300 bundles to build.

The only thing that is unique about this problem project is it was created new with the latest version of Unity. The older projects that use this code all were created using a previous build of Unity.