diff --git a/CHANGELOG.md b/CHANGELOG.md
index e5ac1f6..48bdeab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@ Note: Breaking changes between versions are indicated by "💥".
 
 ## Unreleased
 
+- [Bugfix] Fix "Invalid command argument" during upgrade from Koa to Lilac.
 - [Bugfix] Fix mysql initialisation in docker-compose==2.0.0beta4.
 - [Improvement] Tutor is now published on pypi as "tutor".
 
diff --git a/tutor/commands/k8s.py b/tutor/commands/k8s.py
index dc4ee6c..e884af9 100644
--- a/tutor/commands/k8s.py
+++ b/tutor/commands/k8s.py
@@ -475,7 +475,7 @@ def upgrade_from_koa(config: Config) -> None:
 your MongoDb cluster from v3.6 to v4.0. You should run something similar to:
 
     tutor k8s stop
-    tutor config save --set DOCKER_IMAGE_MONGODB=mongo:4.0
+    tutor config save --set DOCKER_IMAGE_MONGODB=mongo:4.0.25
     tutor k8s start
     tutor k8s exec mongodb mongo --eval 'db.adminCommand({ setFeatureCompatibilityVersion: "4.0" })'
     tutor config save --unset DOCKER_IMAGE_MONGODB
diff --git a/tutor/commands/local.py b/tutor/commands/local.py
index a3f7b71..c6adf7a 100644
--- a/tutor/commands/local.py
+++ b/tutor/commands/local.py
@@ -137,9 +137,9 @@ def upgrade_from_ironwood(context: click.Context, config: Config) -> None:
         )
         return
 
-    upgrade_mongodb(context, config, "3.4")
+    upgrade_mongodb(context, config, "3.4", "3.4")
     context.invoke(compose.stop)
-    upgrade_mongodb(context, config, "3.6")
+    upgrade_mongodb(context, config, "3.6", "3.6")
     context.invoke(compose.stop)
 
 
@@ -183,14 +183,19 @@ def upgrade_from_koa(context: click.Context, config: Config) -> None:
             "nothing left to do to upgrade from Koa to Lilac."
         )
         return
-    upgrade_mongodb(context, config, "4.0.25")
+    upgrade_mongodb(context, config, "4.0.25", "4.0")
 
 
-def upgrade_mongodb(context: click.Context, config: Config, to_version: str) -> None:
-    click.echo(fmt.title("Upgrading MongoDb to v{}".format(to_version)))
+def upgrade_mongodb(
+    context: click.Context,
+    config: Config,
+    to_docker_version: str,
+    to_compatibility_version: str,
+) -> None:
+    click.echo(fmt.title("Upgrading MongoDb to v{}".format(to_docker_version)))
     # Note that the DOCKER_IMAGE_MONGODB value is never saved, because we only save the
     # environment, not the configuration.
-    config["DOCKER_IMAGE_MONGODB"] = "mongo:{}".format(to_version)
+    config["DOCKER_IMAGE_MONGODB"] = "mongo:{}".format(to_docker_version)
     tutor_env.save(context.obj.root, config)
     context.invoke(compose.start, detach=True, services=["mongodb"])
     context.invoke(
@@ -199,7 +204,8 @@ def upgrade_mongodb(context: click.Context, config: Config, to_version: str) ->
             "mongodb",
             "mongo",
             "--eval",
-            'db.adminCommand({ setFeatureCompatibilityVersion: "%s" })' % to_version,
+            'db.adminCommand({ setFeatureCompatibilityVersion: "%s" })'
+            % to_compatibility_version,
         ],
     )
     context.invoke(compose.stop)