diff --git a/vortex-array/src/expr/exprs/fill_null/mod.rs b/vortex-array/src/expr/exprs/fill_null/mod.rs
index 1000747ca95..71affa4c160 100644
--- a/vortex-array/src/expr/exprs/fill_null/mod.rs
+++ b/vortex-array/src/expr/exprs/fill_null/mod.rs
@@ -159,6 +159,8 @@ fn fill_null_canonical(
     if let Some(result) = precondition(canonical.as_ref(), fill_value)? {
         // The result of precondition may return another ScalarFn, in which case we should
         // apply it immediately.
+        // TODO(aduffy): Remove this once we have better driver check. We're also implicitly
+        //  relying on the fact that Cast execution will do an optimize on its result.
         return result.execute::<ArrayRef>(ctx);
     }
     match canonical {