diff --git a/src/Connection/Internal/Http2ConnectionProcessor.php b/src/Connection/Internal/Http2ConnectionProcessor.php
index 7f2686b4..26576e10 100644
--- a/src/Connection/Internal/Http2ConnectionProcessor.php
+++ b/src/Connection/Internal/Http2ConnectionProcessor.php
@@ -1717,6 +1717,8 @@ private function generateHeaders(Request $request): array
                     continue;
                 }
 
+                \assert(is_string($field)); // For Psalm.
+
                 $headers[] = [$field, $value];
             }
         }