diff --git a/doc/HACKING b/doc/HACKING
index b9c53dc56b..51ff60d175 100644
--- a/doc/HACKING
+++ b/doc/HACKING
@@ -68,6 +68,7 @@ for systemd (this example is for Fedora):
         $ meson build                          # configure the build
         $ ninja -C build                       # build it locally, see if everything compiles fine
         $ ninja -C build test                  # run some simple regression tests
+        $ echo root123 > mkosi.rootpw          # set root password used by mkosi
         $ sudo mkosi                           # build a test image
         $ sudo systemd-nspawn -bi image.raw    # boot up the test image
         $ git add -p                           # interactively put together your patch