diff --git a/server/migrations/008_machine_uid.sql b/server/migrations/008_machine_uid.sql
index 8488031..3af40f3 100644
--- a/server/migrations/008_machine_uid.sql
+++ b/server/migrations/008_machine_uid.sql
@@ -34,8 +34,12 @@ ALTER TABLE connect_machines ADD COLUMN IF NOT EXISTS machine_uid TEXT;
 
 -- 2. Enforce one row per machine_uid, but ONLY for rows that actually have one.
 --    A partial UNIQUE index (WHERE machine_uid IS NOT NULL) lets unlimited legacy
---    NULL rows coexist while making a non-null machine_uid a true dedup key — this
---    is what upsert_machine's `ON CONFLICT (machine_uid)` arbiter binds to.
+--    NULL rows coexist while making a non-null machine_uid a true dedup key. Because
+--    this index is PARTIAL, Postgres only binds an ON CONFLICT inference clause to it
+--    when the clause REPEATS the same predicate: upsert_machine's arbiter must be
+--    `ON CONFLICT (machine_uid) WHERE machine_uid IS NOT NULL` (a bare
+--    `ON CONFLICT (machine_uid)` raises "no unique or exclusion constraint matching
+--    the ON CONFLICT specification" at runtime and persists no row).
 CREATE UNIQUE INDEX IF NOT EXISTS idx_connect_machines_machine_uid
     ON connect_machines (machine_uid)
     WHERE machine_uid IS NOT NULL;
diff --git a/server/src/db/machines.rs b/server/src/db/machines.rs
index b6b65d6..901260b 100644
--- a/server/src/db/machines.rs
+++ b/server/src/db/machines.rs
@@ -166,7 +166,7 @@ pub async fn upsert_machine(
                 r#"
                 INSERT INTO connect_machines (agent_id, hostname, is_persistent, status, last_seen, machine_uid)
                 VALUES ($1, $2, $3, 'online', NOW(), $4)
-                ON CONFLICT (machine_uid) DO UPDATE SET
+                ON CONFLICT (machine_uid) WHERE machine_uid IS NOT NULL DO UPDATE SET
                     agent_id = EXCLUDED.agent_id,
                     hostname = EXCLUDED.hostname,
                     status = 'online',