diff --git a/Classes/EventListener/Mapping/EventToListenerMapping.php b/Classes/EventListener/Mapping/EventToListenerMapping.php
index 2d51d0f..305082e 100644
--- a/Classes/EventListener/Mapping/EventToListenerMapping.php
+++ b/Classes/EventListener/Mapping/EventToListenerMapping.php
@@ -71,7 +71,7 @@ public function getOption(string $optionName, $defaultValue)
     /**
      * @inheritDoc
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return [
             'eventClassName' => $this->eventClassName,
diff --git a/Classes/EventListener/Mapping/EventToListenerMappings.php b/Classes/EventListener/Mapping/EventToListenerMappings.php
index 05156ac..717e798 100644
--- a/Classes/EventListener/Mapping/EventToListenerMappings.php
+++ b/Classes/EventListener/Mapping/EventToListenerMappings.php
@@ -84,7 +84,7 @@ public function getIterator(): \Iterator
         return new \ArrayIterator($this->mappings);
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return $this->mappings;
     }
diff --git a/Classes/EventStore/EventStream.php b/Classes/EventStore/EventStream.php
index b5a1cd6..925086e 100644
--- a/Classes/EventStore/EventStream.php
+++ b/Classes/EventStore/EventStream.php
@@ -71,7 +71,7 @@ public function next(): void
         $this->streamIterator->next();
     }
 
-    public function key()
+    public function key(): float|bool|int|string|null
     {
         return $this->streamIterator->key();
     }
diff --git a/Classes/EventStore/Storage/InMemory/InMemoryStreamIterator.php b/Classes/EventStore/Storage/InMemory/InMemoryStreamIterator.php
index 34e9d1f..1229264 100644
--- a/Classes/EventStore/Storage/InMemory/InMemoryStreamIterator.php
+++ b/Classes/EventStore/Storage/InMemory/InMemoryStreamIterator.php
@@ -76,9 +76,9 @@ public function next(): void
     }
 
     /**
-     * @return bool|int
+     * @return bool|int|null
      */
-    public function key()
+    public function key(): bool|int|null
     {
         return $this->innerIterator->valid() ? $this->innerIterator->current()['sequencenumber'] : null;
     }