diff --git a/src/AbstractNumber.php b/src/AbstractNumber.php
index fe53a76..4776922 100644
--- a/src/AbstractNumber.php
+++ b/src/AbstractNumber.php
@@ -544,7 +544,7 @@ public function __toString(): string
     /**
      * Converts the current MadeByBob\Number instance into a string.
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): mixed
     {
         return $this->toString();
     }