diff --git a/src/Common/Promises.php b/src/Common/Promises.php
index b896627..2b93290 100644
--- a/src/Common/Promises.php
+++ b/src/Common/Promises.php
@@ -30,8 +30,8 @@ trait Promises
      */
     protected function addPromise(UuidInterface $uuid, PromiseInterface $promise): self
     {
-        if (! $this->promises->contains($uuid)) {
-            $this->promises->attach($uuid, new ArrayObject());
+        if (! $this->promises->offsetExists($uuid)) {
+            $this->promises->offsetSet($uuid, new ArrayObject());
         }
 
         $this->promises[$uuid][] = $promise;
@@ -60,7 +60,7 @@ protected function addPromise(UuidInterface $uuid, PromiseInterface $promise): s
      */
     protected function removePromise(UuidInterface $uuid, PromiseInterface $promise): self
     {
-        if (! $this->promises->contains($uuid)) {
+        if (! $this->promises->offsetExists($uuid)) {
             throw new InvalidArgumentException(
                 sprintf('There are no registered promises for UUID %s', $uuid->toString())
             );
@@ -96,12 +96,12 @@ protected function removePromise(UuidInterface $uuid, PromiseInterface $promise)
      */
     protected function detachPromises(UuidInterface $uuid): array
     {
-        if (! $this->promises->contains($uuid)) {
+        if (! $this->promises->offsetExists($uuid)) {
             return [];
         }
 
         $promises = $this->promises[$uuid];
-        $this->promises->detach($uuid);
+        $this->promises->offsetUnset($uuid);
 
         return $promises->getArrayCopy();
     }
diff --git a/src/Common/Timers.php b/src/Common/Timers.php
index 2d0641f..325919b 100644
--- a/src/Common/Timers.php
+++ b/src/Common/Timers.php
@@ -27,7 +27,7 @@ trait Timers
      */
     protected function attachTimer(UuidInterface $uuid, TimerInterface $timer): self
     {
-        $this->timers->attach($uuid, $timer);
+        $this->timers->offsetSet($uuid, $timer);
 
         return $this;
     }
@@ -47,13 +47,13 @@ protected function attachTimer(UuidInterface $uuid, TimerInterface $timer): self
      */
     protected function detachTimer(UuidInterface $uuid): ?TimerInterface
     {
-        if (! $this->timers->contains($uuid)) {
+        if (! $this->timers->offsetExists($uuid)) {
             return null;
         }
 
         $timer = $this->timers->offsetGet($uuid);
 
-        $this->timers->detach($uuid);
+        $this->timers->offsetUnset($uuid);
 
         return $timer;
     }
diff --git a/src/Scheduler.php b/src/Scheduler.php
index 25ad3a1..d32d25e 100644
--- a/src/Scheduler.php
+++ b/src/Scheduler.php
@@ -161,7 +161,7 @@ public function remove(Task $task): self
 
         $this->cancelTask($task);
 
-        $this->tasks->detach($task);
+        $this->tasks->offsetUnset($task);
 
         return $this;
     }
@@ -191,7 +191,7 @@ public function removeTasks(): self
      */
     public function hasTask(Task $task): bool
     {
-        return $this->tasks->contains($task);
+        return $this->tasks->offsetExists($task);
     }
 
     /**
@@ -233,7 +233,7 @@ public function schedule(Task $task, Frequency $frequency): self
                     $this->emit(static::ON_TASK_EXPIRED, [$task, $nextDue]);
                 };
 
-                if ($this->promises->contains($task->getUuid())) {
+                if ($this->promises->offsetExists($task->getUuid())) {
                     $pendingPromises = (array) $this->promises->offsetGet($task->getUuid());
                     Promise\all($pendingPromises)->always($removeTask);
                 } else {
@@ -257,7 +257,7 @@ public function schedule(Task $task, Frequency $frequency): self
         );
         $this->emit(static::ON_TASK_SCHEDULED, [$task, $nextDue]);
 
-        $this->tasks->attach($task);
+        $this->tasks->offsetSet($task);
 
         return $this;
     }
diff --git a/tests/Lib/CountableScheduler.php b/tests/Lib/CountableScheduler.php
index 52c3929..ae41c78 100644
--- a/tests/Lib/CountableScheduler.php
+++ b/tests/Lib/CountableScheduler.php
@@ -15,7 +15,7 @@ public function count(): int
 
     public function countPromises(UuidInterface $uuid): int
     {
-        if (! $this->promises->contains($uuid)) {
+        if (! $this->promises->offsetExists($uuid)) {
             return 0;
         }