diff --git a/scripts/image_signing/sign_official_build.sh b/scripts/image_signing/sign_official_build.sh
index 53c6a04..9a1a405 100755
--- a/scripts/image_signing/sign_official_build.sh
+++ b/scripts/image_signing/sign_official_build.sh
@@ -740,10 +740,19 @@
   fi
 
   info "Found ARC image version '${arc_version}', re-signing APKs."
+  # TODO(crbug.com/1141907): remove set -x and set +x below.
+  set -x
   "${SCRIPT_DIR}/sign_android_image.sh" "${rootfs_dir}" "${KEY_DIR}/android"
 
-  sudo umount "${rootfs_dir}"
+  if ! sudo umount "${rootfs_dir}"; then
+    error "umount ${rootfs_dir} failed"
+    sudo lsof -n "${rootfs_dir}"
+    ps auxf
+    set +x
+    return 1
+  fi
   info "Re-signed Android image"
+  set +x
 }
 
 # Sign UEFI binaries, if possible.