diff --git a/src/Curl/Curl.php b/src/Curl/Curl.php
index 8263dd4..279ba02 100644
--- a/src/Curl/Curl.php
+++ b/src/Curl/Curl.php
@@ -1389,7 +1389,8 @@ class Curl extends BaseCurl
                     echo 'Response content length (calculated): ' . $response_calculated_length . "\n";
                 }
 
-                if (preg_match($this->jsonPattern, $this->responseHeaders['Content-Type'])) {
+                if (isset($this->responseHeaders['Content-Type']) &&
+                    preg_match($this->jsonPattern, $this->responseHeaders['Content-Type'])) {
                     $parsed_response = json_decode($this->rawResponse, true);
                     if ($parsed_response !== null) {
                         $messages = [];
diff --git a/tests/PHPCurlClass/PHPCurlClassTest.php b/tests/PHPCurlClass/PHPCurlClassTest.php
index c1fddc7..c57aad2 100644
--- a/tests/PHPCurlClass/PHPCurlClassTest.php
+++ b/tests/PHPCurlClass/PHPCurlClassTest.php
@@ -4363,6 +4363,18 @@ class PHPCurlClassTest extends \PHPUnit\Framework\TestCase
         $this->assertStringContainsString($expect, $test_5_output);
     }
 
+    public function testDiagnoseContentTypeMissing()
+    {
+        $test = new Test();
+        $test->server('json_response', 'POST', [
+            'remove-content-type-header' => '',
+        ]);
+        $test_output = $test->curl->diagnose(true);
+
+        $this->assertFalse(isset($test->curl->responseHeaders['content-type']));
+        $this->assertStringContainsString('Response did not set a content type', $test_output);
+    }
+
     public function testStopRequest()
     {
         $response_length_bytes = 1e6; // 1e6 = 1 megabyte
diff --git a/tests/server.php b/tests/server.php
index 4f1443a..ea1c1bd 100644
--- a/tests/server.php
+++ b/tests/server.php
@@ -217,6 +217,10 @@ if ($test === 'http_basic_auth') {
         ]);
     }
 
+    if (isset($_POST['remove-content-type-header'])) {
+        header_remove('Content-Type');
+    }
+
     echo $body;
     exit;
 } elseif ($test === 'xml_response') {