diff --git a/src/olm/session/mod.rs b/src/olm/session/mod.rs
index 897f7805..a0d038ee 100644
--- a/src/olm/session/mod.rs
+++ b/src/olm/session/mod.rs
@@ -98,7 +98,7 @@ impl ChainStore {
     }
 
     #[cfg(test)]
-    pub fn len(&self) -> usize {
+    pub const fn len(&self) -> usize {
         self.inner.len()
     }