diff --git a/src/yunohost/data_migrations/0021_migrate_to_bullseye.py b/src/yunohost/data_migrations/0021_migrate_to_bullseye.py
index e8f664675..9b11f8084 100644
--- a/src/yunohost/data_migrations/0021_migrate_to_bullseye.py
+++ b/src/yunohost/data_migrations/0021_migrate_to_bullseye.py
@@ -373,6 +373,9 @@ class MyMigration(Migration):
         if free_space_in_directory("/var/") / (1024**3) < 1.0:
             raise YunohostError("migration_0021_not_enough_free_space")
 
+        if free_space_in_directory("/boot/") / (70**3) < 1.0:
+            raise YunohostError("/boot/ has less than 70MB available. This will probably trigger a crash during the upgrade because a new kernel needs to be installed. Please look for advice on the forum on how to remove old unused kernels to free some space in /boot/.", raw_msg=True)
+
         # Check system is up to date
         # (but we don't if 'bullseye' is already in the sources.list ...
         # which means maybe a previous upgrade crashed and we're re-running it)