diff --git a/interim-server/src/routes/relations_single/insert_handler.rs b/interim-server/src/routes/relations_single/insert_handler.rs
index 9148284..48c08b1 100644
--- a/interim-server/src/routes/relations_single/insert_handler.rs
+++ b/interim-server/src/routes/relations_single/insert_handler.rs
@@ -81,7 +81,15 @@ pub(super) async fn post(
     let col_names: Vec<String> = form.keys().cloned().collect();
 
     // Prevent users from modifying Phonograph metadata columns.
-    if col_names.iter().any(|col| col.starts_with('_')) {
+    if form.iter().any(|(col, values)| {
+        col.starts_with('_')
+            && values.iter().any(|value| {
+                serde_json::from_str::<Datum>(value)
+                    .ok()
+                    .map(|value| !value.is_none())
+                    .unwrap_or(false)
+            })
+    }) {
         return Err(forbidden!("access denied to update system metadata column"));
     }