diff --git a/src/rb/local.rs b/src/rb/local.rs
index 40f0496..e751bfa 100644
--- a/src/rb/local.rs
+++ b/src/rb/local.rs
@@ -166,7 +166,7 @@ impl<S: Storage + ?Sized> SplitRef for LocalRb<S> {
     type RefProd<'a> = Prod<&'a Self> where Self: 'a;
     type RefCons<'a> = Cons<&'a Self> where Self: 'a;
 
-    fn split_ref(&mut self) -> (Self::RefProd<'_>, Self::RefCons<'_>) {
+    fn split_ref(&self) -> (Self::RefProd<'_>, Self::RefCons<'_>) {
         (Prod::new(self), Cons::new(self))
     }
 }
diff --git a/src/rb/shared.rs b/src/rb/shared.rs
index abada88..807b39d 100644
--- a/src/rb/shared.rs
+++ b/src/rb/shared.rs
@@ -181,7 +181,7 @@ impl<S: Storage + ?Sized> SplitRef for SharedRb<S> {
     type RefProd<'a> = CachingProd<&'a Self> where Self: 'a;
     type RefCons<'a> = CachingCons<&'a Self> where Self: 'a;
 
-    fn split_ref(&mut self) -> (Self::RefProd<'_>, Self::RefCons<'_>) {
+    fn split_ref(&self) -> (Self::RefProd<'_>, Self::RefCons<'_>) {
         (CachingProd::new(self), CachingCons::new(self))
     }
 }
diff --git a/src/traits/split.rs b/src/traits/split.rs
index f466087..1204dd4 100644
--- a/src/traits/split.rs
+++ b/src/traits/split.rs
@@ -23,5 +23,5 @@ pub trait SplitRef {
         Self: 'a;
 
     /// Perform splitting by reference.
-    fn split_ref(&mut self) -> (Self::RefProd<'_>, Self::RefCons<'_>);
+    fn split_ref(&self) -> (Self::RefProd<'_>, Self::RefCons<'_>);
 }