diff --git a/lib/Assert/Assertion.php b/lib/Assert/Assertion.php
index 81bc978..cfdc63b 100644
--- a/lib/Assert/Assertion.php
+++ b/lib/Assert/Assertion.php
@@ -1950,6 +1950,8 @@ public static function implementsInterface($class, $interfaceName, $message = nu
      */
     public static function isJsonString($value, $message = null, ?string $propertyPath = null): bool
     {
+        static::string($value, $message, $propertyPath);
+
         if (null === \json_decode($value) && JSON_ERROR_NONE !== \json_last_error()) {
             $message = \sprintf(
                 static::generateMessage($message ?: 'Value "%s" is not a valid JSON string.'),
diff --git a/tests/Assert/Tests/AssertTest.php b/tests/Assert/Tests/AssertTest.php
index 849d644..21ce828 100644
--- a/tests/Assert/Tests/AssertTest.php
+++ b/tests/Assert/Tests/AssertTest.php
@@ -1224,6 +1224,13 @@ public function isJsonStringInvalidStringDataprovider(): array
         ];
     }
 
+    public function testIsJsonStringNotString()
+    {
+        $this->expectException('Assert\AssertionFailedException');
+        $this->expectExceptionCode(\Assert\Assertion::INVALID_STRING);
+        Assertion::isJsonString(4);
+    }
+
     /**
      * @dataProvider providesValidUuids
      *