actool fails without any usable diagnostics

[lilyball]

I'm trying to run xcbuild on a project, and I've hit a wall where actool is failing without printing any kind of diagnostic as to why. Here's what I see:

CompileAssetCatalog ./MacVim-cllqusjqgekwejbmyolphrbvwgvq/Build/Products/Release/PSMTabBarControl.bundle/Contents/Resources /private/var/folders/cs/l6c6g06x0974_0w5pdg8dps80000gn/T/nix-build-macvim-8.2.539.drv-0/source/src/MacVim/PSMTabBarControl/Media.xcassets
    cd /private/var/folders/cs/l6c6g06x0974_0w5pdg8dps80000gn/T/nix-build-macvim-8.2.539.drv-0/source/src/MacVim/PSMTabBarControl
    export DEVELOPER_DIR=/nix/store/a99x032swmw9aghn8y4lvhjdimdm7z51-xcodebuild-0.1.2-pre
    export PATH=[long path]
    /nix/store/a99x032swmw9aghn8y4lvhjdimdm7z51-xcodebuild-0.1.2-pre/usr/bin/actool --product-type com.apple.product-type.framework --output-format human-readable-text --notices --warnings --export-dependency-info ./MacVim-cllqusjqgekwejbmyolphrbvwgvq/Build/Intermediates/PSMTabBarControl.build/Release/PSMTabBarControlFramework.build/assetcatalog_dependencies --output-partial-info-plist ./MacVim-cllqusjqgekwejbmyolphrbvwgvq/Build/Intermediates/PSMTabBarControl.build/Release/PSMTabBarControlFramework.build/assetcatalog_generated_info.plist --target-device mac --sticker-pack-identifier-prefix com.positivespinmedia.PSMTabBarControlFramework.sticker-pack. --compile ./MacVim-cllqusjqgekwejbmyolphrbvwgvq/Build/Products/Release/PSMTabBarControl.bundle/Contents/Resources /private/var/folders/cs/l6c6g06x0974_0w5pdg8dps80000gn/T/nix-build-macvim-8.2.539.drv-0/source/src/MacVim/PSMTabBarControl/Media.xcassets --platform macosx --minimum-deployment-target 10.12
/* com.apple.actool.errors */
: error: unable to create compiled asset writer

/* com.apple.actool.warnings */
: warning: product type not supported
: warning: platform not supported
: warning: sticker pack not supported
: warning: target device not supported

The warnings seem harmless; it's targeting a mac framework. "sticker pack" is just because of the --sticker-pack-identifier-prefix flag, it's not actually building a sticker pack.

But then, it just fails, saying "unable to create compiled asset writer". No explanation as to why not.

As best I can figure, that means CreateWriter failed, but there's no info as to why it failed or what I can do about this to move forward.