#19979 mysteriously terse engine error message when docker image_tags=[] Issue created by cburroughs Describe the bug Starting with https://github.com/pantsbuild/example-docker

$ git diff | cat
diff --git a/src/docker/hello_world/BUILD b/src/docker/hello_world/BUILD
index 4b6c1c4..83e04a9 100644
--- a/src/docker/hello_world/BUILD
+++ b/src/docker/hello_world/BUILD
@@ -4,6 +4,7 @@
 docker_image(
     name="python",
     source="Dockerfile.python",
+    image_tags = []
 )
 
 docker_image(
$ pants package src/docker/hello_world:python
15:32:12.80 [ERROR] 1 Exception encountered:

Engine traceback:
  in `package` goal

IndexError: tuple index out of range
$ ls -lsthra .pants.d/
total 16K
 512 -rw-r----- 1 ecsb ecsb    0 Oct  4 15:30 exceptions.log
 512 -rw-r----- 1 ecsb ecsb    0 Oct  4 15:30 exceptions.4637.log
1.5K drwxr-x--- 3 ecsb ecsb    6 Oct  4 15:30 .
1.5K drwxr-x--- 8 ecsb ecsb   15 Oct  4 15:30 ..
 11K drwxr-x--- 6 ecsb ecsb    6 Oct  4 15:32 run-tracker
1.5K -rw-r----- 1 ecsb ecsb 2.7K Oct  4 15:32 pants.log
$ pants package -ltrace src/docker/hello_world:python
15:32:41.40 [INFO] Initialization options changed: reinitializing scheduler...
15:32:47.64 [INFO] Scheduler initialized.
15:32:49.55 [ERROR] 1 Exception encountered:

Engine traceback:
  in `package` goal

IndexError: tuple index out of range

Note that the exception log files are empty and logging doesn't give any additional clues. Pants version 2.17.0 OS Linux Additional info I ran into this with a macro that generated image tags and left them blank in some not-in-CI cases. python tests would then throw the above error locally due to a

runtime_package_dependencies

on said image. Which is all to say that the cause was not obvious. pantsbuild/pants