diff --git a/libraries/persistent_store/src/lib.rs b/libraries/persistent_store/src/lib.rs
index 4be15e3..0ff3224 100644
--- a/libraries/persistent_store/src/lib.rs
+++ b/libraries/persistent_store/src/lib.rs
@@ -376,6 +376,8 @@ pub use self::store::{
 /// with Rust, `usize` is used instead of `Nat` in code meant only for tests.
 ///
 /// Currently, the store only supports targets with `usize = u32`.
+// Make sure production builds have `usize = 32`.
+#[cfg(any(target_pointer_width = "32", feature = "std"))]
 type Nat = u32;
 
 /// Returns the internal representation of a Rust natural number.