Error "invalid context directory"

What means this error?

docker-slim[build]: info=basic.image.name value=docker-slim-tmp-fat-image.15089.20200625042508 time="2020-06-25T07:25:08+03:00" level=fatal msg="docker-slim: failure" error="invalid context directory" stack="goroutine 1 [running]:\nruntime/debug.Stack(0xf506d8, 0xc000062f70, 0xc00047d160)\n\truntime/debug/stack.go:24 +0x9d\ngithub.com/docker-slim/docker-slim/pkg/util/errutil.FailOn(0xb9ab20, 0xc00001f6d0)\n\tgithub.com/docker-slim/docker-slim/pkg/util/errutil/errutil.go:14 +0x54\ngithub.com/docker-slim/docker-slim/internal/app/master/commands.OnBuild(0xc00005c0f0, 0xc000026cf0, 0x23, 0xc000163770, 0xc, 0x0, 0x0, 0x0, 0x0, 0xa11d01, ...)\n\tgithub.com/docker-slim/docker-slim/internal/app/master/commands/build.go:139 +0x561b\ngithub.com/docker-slim/docker-slim/internal/app/master.init.0.func10(0xc000098160, 0x0, 0xc0002bef00)\n\tgithub.com/docker-slim/docker-slim/internal/app/master/cli.go:1655 +0x171a\ngithub.com/urfave/cli.HandleAction(0x9cd840, 0xad0bd8, 0xc000098160, 0xc000098160, 0x0)\n\tgithub.com/urfave/cli@v1.22.1/app.go:523 +0x11a\ngithub.com/urfave/cli.Command.Run(0xaa0a83, 0x5, 0x0, 0x0, 0xc000130ff0, 0x1, 0x1, 0xac41f6, 0x3e, 0x0, ...)\n\tgithub.com/urfave/cli@v1.22.1/command.go:174 +0x56e\ngithub.com/urfave/cli.(*App).Run(0xc000166000, 0xc0000b8180, 0x8, 0x8, 0x0, 0x0)\n\tgithub.com/urfave/cli@v1.22.1/app.go:276 +0x7ae\ngithub.com/docker-slim/docker-slim/internal/app/master.(*InteractiveApp).execute(0xc000100ea0, 0xc00033e000, 0x5f)\n\tgithub.com/docker-slim/docker-slim/internal/app/master/cli.go:328 +0x3c4\ngithub.com/c-bata/go-prompt.(*Prompt).Run(0xc0000b8680)\n\tgithub.com/c-bata/go-prompt@v0.2.3/prompt.go:85 +0x814\ngithub.com/docker-slim/docker-slim/internal/app/master.(*InteractiveApp).Run(...)\n\tgithub.com/docker-slim/docker-slim/internal/app/master/cli.go:414\ngithub.com/docker-slim/docker-slim/internal/app/master.init.0.func3(0xc000098dc0, 0x0, 0x0)\n\tgithub.com/docker-slim/docker-slim/internal/app/master/cli.go:951 +0x70\ngithub.com/urfave/cli.HandleAction(0x9cd840, 0xad0bf8, 0xc000098dc0, 0xc000098dc0, 0x0)\n\tgithub.com/urfave/cli@v1.22.1/app.go:523 +0x11a\ngithub.com/urfave/cli.(*App).Run(0xc000166000, 0xc00001e1d0, 0x1, 0x1, 0x0, 0x0)\n\tgithub.com/urfave/cli@v1.22.1/app.go:285 +0x630\ngithub.com/docker-slim/docker-slim/internal/app/master.runCli()\n\tgithub.com/docker-slim/docker-slim/internal/app/master/cli.go:2053 +0x55\ngithub.com/docker-slim/docker-slim/internal/app/master.Run()\n\tgithub.com/docker-slim/docker-slim/internal/app/master/app.go:6 +0x25\nmain.main()\n\tgithub.com/docker-slim/docker-slim/cmd/docker-slim/main.go:8 +0x20\n" version="linux|Transformer|1.29.0|afff2c9679a697ebfc933360267253a10325269d|2020-03-18_07:11:20PM"

Start at internal/app/master/commands/build.go:139 in OnBuild, the location identified by the stack trace. Reproduce the reported build failure with the same inputs and trace which context validation emits the error. Done when the cause and clear corrective guidance are documented, with a regression test if the project has one.