diff --git a/src/IO/JSON/JSONIOHandlerImpl.cpp b/src/IO/JSON/JSONIOHandlerImpl.cpp
index 72133a7db6..c10d64934f 100644
--- a/src/IO/JSON/JSONIOHandlerImpl.cpp
+++ b/src/IO/JSON/JSONIOHandlerImpl.cpp
@@ -151,7 +151,7 @@ namespace
                 auto asToml = openPMD::json::jsonToToml(shadow);
                 std::cerr << "Warning: parts of the backend configuration for "
                              "JSON/TOML backend remain unused:\n"
-                          << asToml << std::endl;
+                          << json::format_toml(asToml) << std::endl;
                 break;
             }
             }