diff --git a/dist/platforms/ubuntu/steps/return_license.sh b/dist/platforms/ubuntu/steps/return_license.sh
index 70006dd..45782aa 100644
--- a/dist/platforms/ubuntu/steps/return_license.sh
+++ b/dist/platforms/ubuntu/steps/return_license.sh
@@ -106,12 +106,20 @@ elif [[ "$RETURN_STRATEGY" == "serial" ]]; then
   # project, so Unity doesn't reopen the real project (and reimport its
   # library against whatever the editor's default target is) just to
   # return the license (game-ci/cli#33).
+  #
+  # -username/-password mirror the serial branch of activate.sh, and Unity
+  # documents them on -returnlicense exactly as on activation:
+  # https://docs.unity3d.com/Manual/ManagingYourUnityLicense.html (documented identically
+  # for every Unity line game-ci tests: 2018.4, 2019.4, 2020.3, 2022.3, 6000.x)
+  # Without them the returning editor starts a fresh licensing client that then fails to return the license.
   RETURN_LOG="$(mktemp)"
   for ATTEMPT in $(seq 1 "$UNITY_LICENSE_RETURN_MAX_ATTEMPTS"); do
     unity-editor \
       -logFile /dev/stdout \
       -quit \
       -returnlicense \
+      -username "$UNITY_EMAIL" \
+      -password "$UNITY_PASSWORD" \
       -projectPath "$ACTIVATE_LICENSE_PATH" 2>&1 | tee "$RETURN_LOG"
     RETURN_EXIT_CODE=${PIPESTATUS[0]}
 
diff --git a/scripts/test-licensing-steps.sh b/scripts/test-licensing-steps.sh
index 3a09442..48d4e67 100644
--- a/scripts/test-licensing-steps.sh
+++ b/scripts/test-licensing-steps.sh
@@ -288,6 +288,21 @@ run_step UNITY_LICENSING_METHOD="serial" UNITY_SERIAL="F4-XXXX" UNITY_LICENSING_
 check "an explicit strategy governs the return" "$(cat "$ARGV_LOG")" "-returnlicense"
 refute "and does not fall back to floating" "$(cat "$ARGV_LOG")" "--return-floating"
 
+# A serial-mode return has to authenticate, exactly like the activation it
+# undoes. Without credentials the returning editor starts a fresh licensing
+# client with no session and reports "Access token is unavailable; failed to
+# update", then "Failed to return entitlement license" - and because that
+# string is in the transient pattern above, it is retried through the full
+# backoff before the seat is leaked anyway. mac/steps/return_license.sh and
+# both windows return scripts have always passed these; the ubuntu steps
+# script lost them, which is what this asserts can not happen again.
+: > "$ARGV_LOG"
+run_step UNITY_LICENSING_METHOD="serial" UNITY_SERIAL="F4-XXXX" \
+  UNITY_EMAIL="ci@example.com" UNITY_PASSWORD="pw123456" \
+  bash -c 'source "$STEPS_DIR/return_license.sh"' > /dev/null 2>&1
+check "a serial return authenticates with the account username" "$(cat "$ARGV_LOG")" "-username ci@example.com"
+check "a serial return authenticates with the account password" "$(cat "$ARGV_LOG")" "-password pw123456"
+
 echo "Ambiguous licensing method warning"
 # A credential that names a specific strategy (a .ulf, a real serial, a
 # licensing server) but gets silently overridden by a different strategy is