diff --git a/state/redis/README.md b/state/redis/README.md
index 6729c2b..a62a308 100644
--- a/state/redis/README.md
+++ b/state/redis/README.md
@@ -69,14 +69,10 @@ with its own `go.mod` [go.mod:module]. `redis.go` imports
   the Go methods return `false` [redis.go:extendScript]
   [redis.go:releaseScript] [redis.go:State.ExtendLock]
   [redis.go:State.ReleaseLock].
-- `ExtendLock` passes `ttl.Milliseconds()` to the script, so it truncates
-  sub-millisecond precision [redis.go:State.ExtendLock].
-- A positive `ttl` under 1 ms therefore reaches the script as `0`. Redis
-  deletes a key given a non-positive `PEXPIRE` and returns `1`, so
-  `ExtendLock` deletes the lock and still returns `true`
-  [redis.go:State.ExtendLock] [redis.go:extendScript]. The runtime renews
-  with `ThreadLockTTL` (default 2 minutes), so only a sub-millisecond
-  configuration reaches this path.
+- `ExtendLock` passes `ttl` to the script in whole milliseconds, so it
+  truncates sub-millisecond precision. A positive `ttl` under 1 ms becomes
+  1 ms, as go-redis does for `SetNX`, so `PEXPIRE` never gets `0` and never
+  deletes the lock [redis.go:State.ExtendLock] [redis.go:extendScript].
 - Redis errors from the state methods are returned unwrapped. `New` wraps a
   ping error as `"redis state: ping: %w"` [redis.go:New].
 - `Shutdown` closes the client at most once. It returns the `Close` error from
@@ -97,7 +93,7 @@ with its own `go.mod` [go.mod:module]. `redis.go` imports
   with the token and the caller's `ttl` [redis.go:State.AcquireLock]
   [redis.go:newToken].
 - `ExtendLock` sends `EVAL extendScript` on `<prefix>:lock:<key>` with
-  `ARGV = [token, ttl in ms]` [redis.go:State.ExtendLock].
+  `ARGV = [token, max(ttl in ms, 1)]` [redis.go:State.ExtendLock].
 - `ReleaseLock` sends `EVAL releaseScript` on `<prefix>:lock:<key>` with
   `ARGV = [token]` [redis.go:State.ReleaseLock].
 - `Shutdown` calls `Close` on the caller-supplied `redis.UniversalClient`
diff --git a/state/redis/redis.go b/state/redis/redis.go
index 9067b98..b776f51 100644
--- a/state/redis/redis.go
+++ b/state/redis/redis.go
@@ -109,7 +109,7 @@ func (s *State) ExtendLock(ctx context.Context, lease chat.LockLease, ttl time.D
 	if ttl <= 0 {
 		return false, errors.New("redis state: lock ttl must be positive")
 	}
-	result, err := s.client.Eval(ctx, extendScript, []string{s.key("lock", lease.Key)}, lease.Token, ttl.Milliseconds()).Int()
+	result, err := s.client.Eval(ctx, extendScript, []string{s.key("lock", lease.Key)}, lease.Token, max(ttl.Milliseconds(), 1)).Int()
 	if err != nil {
 		return false, err
 	}
diff --git a/state/redis/redis_test.go b/state/redis/redis_test.go
index 5a5cef3..5c141d2 100644
--- a/state/redis/redis_test.go
+++ b/state/redis/redis_test.go
@@ -38,6 +38,39 @@ func TestStateConformance(t *testing.T) {
 	})
 }
 
+func TestExtendLockSubMillisecondTTLKeepsLock(t *testing.T) {
+	server := miniredis.RunT(t)
+	client := goredis.NewClient(&goredis.Options{Addr: server.Addr()})
+	t.Cleanup(func() {
+		_ = client.Close()
+	})
+	ctx := context.Background()
+	state, err := chatredis.New(ctx, chatredis.Options{Client: client, Prefix: "test"})
+	if err != nil {
+		t.Fatalf("new redis state: %v", err)
+	}
+
+	for _, ttl := range []time.Duration{time.Nanosecond, 500 * time.Microsecond} {
+		t.Run(ttl.String(), func(t *testing.T) {
+			key := "thread-" + ttl.String()
+			lease, acquired, err := state.AcquireLock(ctx, key, time.Minute)
+			if err != nil || !acquired {
+				t.Fatalf("AcquireLock = %v, %v; want true, nil", acquired, err)
+			}
+			extended, err := state.ExtendLock(ctx, lease, ttl)
+			if err != nil || !extended {
+				t.Fatalf("ExtendLock = %v, %v; want true, nil", extended, err)
+			}
+			if !server.Exists("test:lock:" + key) {
+				t.Fatal("lock key was deleted by ExtendLock")
+			}
+			if _, acquired, err := state.AcquireLock(ctx, key, time.Minute); err != nil || acquired {
+				t.Fatalf("second AcquireLock = %v, %v; want false, nil", acquired, err)
+			}
+		})
+	}
+}
+
 func TestStateConformanceWithRedisContainer(t *testing.T) {
 	testcontainers.SkipIfProviderIsNotHealthy(t)