diff --git a/package_manager/src/ComposerInspector.php b/package_manager/src/ComposerInspector.php
index 3cab289969ae27df4258c5627097aba0ca073139..2c41504880c049237cdf8bbfa7dee285ae818b70 100644
--- a/package_manager/src/ComposerInspector.php
+++ b/package_manager/src/ComposerInspector.php
@@ -76,7 +76,9 @@ final class ComposerInspector {
        * {@inheritdoc}
        */
       public function __invoke(string $type, string $buffer): void {
-        $this->output .= trim($buffer);
+        if ($type === ProcessOutputCallbackInterface::OUT) {
+          $this->output .= trim($buffer);
+        }
       }
 
     };