diff --git a/src/Result/AbstractResult.php b/src/Result/AbstractResult.php
index 95416dd..54c0b7c 100644
--- a/src/Result/AbstractResult.php
+++ b/src/Result/AbstractResult.php
@@ -19,24 +19,24 @@ public function getData(): array
         return $this->data;
     }
 
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         $data = $this->getData();
         return array_key_exists($offset, $data);
     }
 
-    public function offsetGet($offset)
+    public function offsetGet($offset): mixed
     {
         $data = $this->getData();
         return $data[$offset] ?? null;
     }
 
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         throw new \RuntimeException('You should not change the data in a result.');
     }
 
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         throw new \RuntimeException('You should not change the data in a result.');
     }
diff --git a/src/Result/Invoice.php b/src/Result/Invoice.php
index 37f07b2..62d0e87 100644
--- a/src/Result/Invoice.php
+++ b/src/Result/Invoice.php
@@ -83,7 +83,7 @@ public function isNew(): bool
         return $data['status'] === self::STATUS_NEW;
     }
 
-    public function isFullyPaid(): bool
+    public function isSettled(): bool
     {
         $data = $this->getData();
         return $data['status'] === self::STATUS_SETTLED;