diff --git a/ethexe/runtime/common/src/state.rs b/ethexe/runtime/common/src/state.rs
index facbe376b96..af14882cf08 100644
--- a/ethexe/runtime/common/src/state.rs
+++ b/ethexe/runtime/common/src/state.rs
@@ -899,6 +899,8 @@ impl IndexMut<RegionIdx> for MemoryPages {
     }
 }
 
+/// An inner structure for [`MemoryPages`]. Has at most 256 entries because
+/// [`RegionIdx`] is `u8` internally.
 pub type MemoryPagesInner = [MaybeHashOf<MemoryPagesRegion>; 256];
 
 impl MemoryPages {