diff --git a/src/iface/neighbor.rs b/src/iface/neighbor.rs
index 871c923..acebba6 100644
--- a/src/iface/neighbor.rs
+++ b/src/iface/neighbor.rs
@@ -103,6 +103,7 @@ impl<'a> Cache<'a> {
                             .0
                     }
                     // Owned maps can extend themselves.
+                    #[cfg(any(feature = "std", feature = "alloc"))]
                     ManagedMap::Owned(_) => unreachable!()
                 };