diff --git a/lib/Http/Client/Middleware/HttpClientLoggerMiddleware.php b/lib/Http/Client/Middleware/HttpClientLoggerMiddleware.php
index ce3db81..c46e785 100644
--- a/lib/Http/Client/Middleware/HttpClientLoggerMiddleware.php
+++ b/lib/Http/Client/Middleware/HttpClientLoggerMiddleware.php
@@ -270,7 +270,9 @@ function ($reason) use ($request, $reqHeaders, $reqId) {
 					if ($reason instanceof \Throwable) {
 						throw $reason;
 					}
-					throw new \RuntimeException('HTTP request rejected: ' . (string)$reason);
+					throw new \RuntimeException(
+						'HTTP request rejected: ' . (is_object($reason) ? get_class($reason) : (string)$reason)
+					);
 				}
 			);
 		};
diff --git a/lib/Http/Client/Middleware/LogWriter.php b/lib/Http/Client/Middleware/LogWriter.php
index 082b8ba..5ad6a20 100644
--- a/lib/Http/Client/Middleware/LogWriter.php
+++ b/lib/Http/Client/Middleware/LogWriter.php
@@ -28,7 +28,12 @@ public static function write(
 		LoggerInterface $logger,
 	): void {
 		if (in_array($format, ['json', 'both'], true)) {
-			self::append($jsonFile, json_encode($entry, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE) . PHP_EOL, $logger);
+			// Remote servers can send headers with broken UTF-8; substitute
+			// those bytes instead of losing the whole entry.
+			$json = json_encode($entry, JSON_UNESCAPED_SLASHES | JSON_UNESCAPED_UNICODE | JSON_INVALID_UTF8_SUBSTITUTE);
+			if ($json !== false) {
+				self::append($jsonFile, $json . PHP_EOL, $logger);
+			}
 		}
 		if (in_array($format, ['plain', 'both'], true)) {
 			self::append($plainFile, $plainLine, $logger);