diff --git a/crates/fayalite/src/sim.rs b/crates/fayalite/src/sim.rs
index 275b106..b508756 100644
--- a/crates/fayalite/src/sim.rs
+++ b/crates/fayalite/src/sim.rs
@@ -6698,6 +6698,11 @@ impl SimulationModuleState {
         mut target: Target,
         which_module: WhichModule,
     ) -> CompiledValue<CanonicalType> {
+        assert!(
+            target.canonical_ty().is_passive(),
+            "simulator read/write expression must have a passive type \
+            (recursively contains no fields with `#[hdl(flip)]`)"
+        );
         if let Some(&retval) = self.io_targets.get(&target) {
             return retval;
         }