diff -r c0301da04883 -r 7b2ee3e84c5f src/bisection_tree/refine.rs
--- a/src/bisection_tree/refine.rs	Fri Dec 06 15:30:23 2024 -0500
+++ b/src/bisection_tree/refine.rs	Fri Dec 06 16:14:41 2024 -0500
@@ -364,8 +364,13 @@
                                 let mut container = container_arc.lock().unwrap();
                                 // Safe: we just created arg_b and have a mutable exclusive
                                 // reference to self containing it.
+                                #[cfg(feature = "nightly")]
                                 unsafe { Arc::get_mut_unchecked(arc_b) }
                                     .stage_refine(domain, &mut *container);
+                                #[cfg(not(feature = "nightly"))]
+                                Arc::get_mut(arc_b).unwrap()
+                                    .stage_refine(domain, &mut *container);
+                                
                                 return Err(container)
                             },
                             _ => unreachable!("This cannot happen"),