diff --git a/crates/clawhdf5-format/src/chunked_read.rs b/crates/clawhdf5-format/src/chunked_read.rs
index 712aab8..ddadda7 100644
--- a/crates/clawhdf5-format/src/chunked_read.rs
+++ b/crates/clawhdf5-format/src/chunked_read.rs
@@ -155,7 +155,9 @@ impl ChunkPlacer {
             &mut long
         };
         for (o, &off) in chunk_offsets.iter_mut().zip(offsets) {
-            *o = off as usize;
+            // An offset past `usize` is past the dataset: the chunk writes
+            // nothing (a plain cast would wrap it into the output).
+            *o = usize::try_from(off).unwrap_or(usize::MAX);
         }
         // SAFETY: the caller's contract.
         unsafe {