diff --git a/src/Http/GuzzleHttpClient.php b/src/Http/GuzzleHttpClient.php
index 8bee107..0fe803b 100644
--- a/src/Http/GuzzleHttpClient.php
+++ b/src/Http/GuzzleHttpClient.php
@@ -467,7 +467,7 @@ private function buildApiException(
 
         if (!$parsedEnvelope) {
             // HTTP layer succeeded, body is unparseable as APIError.
-            $message = 'failed to parse error response';
+            $message = sprintf('failed to parse error response: unexpected server response code %d', $statusCode);
         }
 
         if ($statusCode === 429) {
diff --git a/tests/Exceptions/ErrorHandlingTest.php b/tests/Exceptions/ErrorHandlingTest.php
index 2116157..3680122 100644
--- a/tests/Exceptions/ErrorHandlingTest.php
+++ b/tests/Exceptions/ErrorHandlingTest.php
@@ -106,7 +106,7 @@ public function unparseableErrorResponseFallsBackToSentinelMessage(): void
             self::assertSame(500, $e->getStatusCode());
             self::assertSame(500, $e->getCode(), 'getCode() returns HTTP status (back-compat)');
             self::assertSame(0, $e->getApiErrorCode(), 'APIError.code is 0 on unparseable body');
-            self::assertSame('failed to parse error response', $e->getMessage());
+            self::assertSame('failed to parse error response: unexpected server response code 500', $e->getMessage());
             self::assertSame('<<<not json>>>', $e->getRawResponseBody());
             self::assertNotNull($e->getPrevious(), 'cause chain must point to the JSON parse error');
             self::assertInstanceOf(\JsonException::class, $e->getPrevious());