diff --git a/tvix/castore/src/composition.rs b/tvix/castore/src/composition.rs
index 3550695a8..c76daafc5 100644
--- a/tvix/castore/src/composition.rs
+++ b/tvix/castore/src/composition.rs
@@ -149,7 +149,7 @@ impl<'r, 'de: 'r, T: 'static> SeedFactory<'de, TagString<'de>> for RegistryWithF
              .0
             .iter()
             .find(|(k, _)| *k == &(TypeId::of::<T>(), tag.as_ref()))
-            .ok_or_else(|| serde::de::Error::custom("Unknown tag"))?
+            .ok_or_else(|| serde::de::Error::custom(format!("Unknown type: {}", tag)))?
             .1;
 
         let entry: &RegistryEntry<T> = <dyn Any>::downcast_ref(&**seed).unwrap();
@@ -215,7 +215,7 @@ impl<'de, T: 'static> serde::Deserialize<'de> for DeserializeWithRegistry<T> {
 
 #[derive(Debug, thiserror::Error)]
 enum TryFromUrlError {
-    #[error("Unknown tag: {0}")]
+    #[error("Unknown type: {0}")]
     UnknownTag(String),
 }