diff --git a/src/http/include/sourcemeta/hydra/http_stream.h b/src/http/include/sourcemeta/hydra/http_stream.h
index 10976058..223814f3 100644
--- a/src/http/include/sourcemeta/hydra/http_stream.h
+++ b/src/http/include/sourcemeta/hydra/http_stream.h
@@ -45,7 +45,7 @@ class SOURCEMETA_HYDRA_HTTP_EXPORT Stream {
   auto on_header(HeaderCallback callback) noexcept -> void;
 
 private:
-  struct Internal;
+  struct SOURCEMETA_HYDRA_HTTP_EXPORT Internal;
 
   // No need to make this private, as the contents of `Internal`
   // are already hidden with the PIMPL idiom.