diff --git a/src/Extend/ConcreteExtensionStore.php b/src/Extend/ConcreteExtensionStore.php
index 5d267fde..0825bab8 100644
--- a/src/Extend/ConcreteExtensionStore.php
+++ b/src/Extend/ConcreteExtensionStore.php
@@ -106,7 +106,7 @@ private static function extendOrReplace(SelectorList $selector, SelectorList $so
 
         if (!$selector->isInvisible()) {
             foreach ($selector->getComponents() as $component) {
-                $extender->originals->attach($component);
+                $extender->originals->offsetSet($component);
             }
         }
 
@@ -224,7 +224,7 @@ public function addSelector(SelectorList $selector, ?array $mediaContext): Box
 
         if (!$originalSelector->isInvisible()) {
             foreach ($originalSelector->getComponents() as $component) {
-                $this->originals->attach($component);
+                $this->originals->offsetSet($component);
             }
         }
 
@@ -239,7 +239,7 @@ public function addSelector(SelectorList $selector, ?array $mediaContext): Box
         $modifiableSelector = new ModifiableBox($selector);
 
         if ($mediaContext !== null) {
-            $this->mediaContexts->attach($modifiableSelector, $mediaContext);
+            $this->mediaContexts->offsetSet($modifiableSelector, $mediaContext);
         }
 
         $this->registerSelector($selector, $modifiableSelector);
@@ -261,7 +261,7 @@ private function registerSelector(SelectorList $list, ModifiableBox $selector):
                     if (!isset($this->selectors[$simple])) {
                         /** @var ObjectSet<ModifiableBox<SelectorList>> $set */
                         $set = new ObjectSet();
-                        $this->selectors->attach($simple, $set);
+                        $this->selectors->offsetSet($simple, $set);
                     }
                     $this->selectors[$simple]->add($selector);
 
@@ -422,7 +422,7 @@ private function extendExistingExtensions(array $extensions, SimpleSelectorMap $
                         }
                     }
 
-                    if ($newExtensions->contains($extension->target)) {
+                    if ($newExtensions->offsetExists($extension->target)) {
                         /** @var SimpleSelectorMap<ComplexSelectorMap<Extension>> $additionalExtensions */
                         $additionalExtensions ??= new SimpleSelectorMap();
 
@@ -585,7 +585,7 @@ private function extendList(SelectorList $list, SimpleSelectorMap $extensions, ?
             return $list;
         }
 
-        return new SelectorList($this->trim($extended, $this->originals->contains(...)), $list->getSpan());
+        return new SelectorList($this->trim($extended, $this->originals->offsetExists(...)), $list->getSpan());
     }
 
     /**
@@ -618,7 +618,7 @@ private function extendComplex(ComplexSelector $complex, SimpleSelectorMap $exte
         //     ]
         //
         $extendedNotExpanded = null;
-        $isOriginal = $this->originals->contains($complex);
+        $isOriginal = $this->originals->offsetExists($complex);
 
         foreach ($complex->getComponents() as $i => $component) {
             $extended = $this->extendCompound($component, $extensions, $mediaQueryContext, $isOriginal);
@@ -684,8 +684,8 @@ private function extendComplex(ComplexSelector $complex, SimpleSelectorMap $exte
                 // Make sure that copies of $complex retain their status as "original"
                 // selectors. This includes selectors that are modified because a :not()
                 // was extended into.
-                if ($first && $this->originals->contains($complex)) {
-                    $this->originals->attach($outputComplex);
+                if ($first && $this->originals->offsetExists($complex)) {
+                    $this->originals->offsetSet($outputComplex);
                 }
 
                 $first = false;
@@ -917,7 +917,7 @@ private function extendSimple(SimpleSelector $simple, SimpleSelectorMap $extensi
             if ($extensionsForSimple === null) {
                 return null;
             }
-            $targetsUsed?->attach($simple);
+            $targetsUsed?->offsetSet($simple);
 
             $result = [];
 
diff --git a/src/Extend/ObjectSet.php b/src/Extend/ObjectSet.php
index 2a122b60..fc18a6f5 100644
--- a/src/Extend/ObjectSet.php
+++ b/src/Extend/ObjectSet.php
@@ -33,7 +33,7 @@ public function __construct()
      */
     public function contains(object $value): bool
     {
-        return $this->storage->contains($value);
+        return $this->storage->offsetExists($value);
     }
 
     /**
@@ -41,7 +41,7 @@ public function contains(object $value): bool
      */
     public function add(object $value): void
     {
-        $this->storage->attach($value);
+        $this->storage->offsetSet($value);
     }
 
     /**