diff --git a/corsika/detail/modules/ObservationPlane.inl b/corsika/detail/modules/ObservationPlane.inl
index 0d2ea0639db1c7f7f877d3ad2a3a9c8ae6d59d65..99d416bc3ff7f566e9ed98ebd194e2d15c431bde 100644
--- a/corsika/detail/modules/ObservationPlane.inl
+++ b/corsika/detail/modules/ObservationPlane.inl
@@ -118,7 +118,7 @@ namespace corsika {
 
     // basic info
     node["type"] = "ObservationPlane";
-    node["units"] = "m"; // add default units for values
+    node["units"]["length"] = "m"; // add default units for values
 
     // the center of the plane
     auto const center{plane_.getCenter()};