diff --git a/template/core/classes/class_response_parser.php b/template/core/classes/class_response_parser.php
index 3f493704..34db0945 100644
--- a/template/core/classes/class_response_parser.php
+++ b/template/core/classes/class_response_parser.php
@@ -232,9 +232,9 @@ public function parseString(string $input): null|<?php echo PHPFHIR_INTERFACE_TY
             return null;
         }
         $chr = $input[0];
-        if (in_array(self::XML_START, $chr, true)) {
+        if (in_array($chr, self::XML_START, true)) {
             return $this->parseXml($input);
-        } elseif (in_array(self::JSON_START, $chr, true)) {
+        } elseif (in_array($chr, self::JSON_START, true)) {
             return $this->parseJson($input);
         }
         throw new \UnexpectedValueException(sprintf(