diff --git a/misc/docker/dock.go b/misc/docker/dock.go
index 769d91224..16228818b 100644
--- a/misc/docker/dock.go
+++ b/misc/docker/dock.go
@@ -45,7 +45,7 @@ import (
 
 var (
 	flagRev     = flag.String("rev", "", "Camlistore revision to build (tag or commit hash). For development purposes, you can instead specify the path to a local Camlistore source tree from which to build, with the form \"WIP:/path/to/dir\".")
-	flagVersion = flag.String("tarball_version", "", "Version number (e.g. 0.9) used for the release tarball name. It also defines the destination directory where the release tarball is uploaded.")
+	flagVersion = flag.String("tarball_version", "", "For --build_image mode, the version number (e.g. 0.9) used for the release tarball name. It also defines the destination directory where the release tarball is uploaded.")
 	buildOS     = flag.String("os", runtime.GOOS, "Operating system to build for. Requires --build_release.")
 
 	doImage    = flag.Bool("build_image", true, "build the Camlistore server as a docker image. Conflicts with --build_release.")
@@ -452,6 +452,10 @@ func checkFlags() {
 		fmt.Fprintf(os.Stderr, "Usage error: --tarball_version required when building the release tarball.\n")
 		usage()
 	}
+	if *doImage && *flagVersion != "" {
+		fmt.Fprintf(os.Stderr, "Usage error: --tarball_version not applicable in --build_image mode.\n")
+		usage()
+	}
 	if isWIP() {
 		if _, err := os.Stat(localCamliSource()); err != nil {
 			fmt.Fprintf(os.Stderr, "Usage error: could not stat path %q provided with --rev: %v", localCamliSource(), err)