diff --git a/hugr/src/ops/constant/custom.rs b/hugr/src/ops/constant/custom.rs
index 58b5de4bf..6a1b84f45 100644
--- a/hugr/src/ops/constant/custom.rs
+++ b/hugr/src/ops/constant/custom.rs
@@ -312,7 +312,8 @@ pub fn get_pair_of_input_values<T: CustomConst>(
     Some((c0.get_custom_value()?, c1.get_custom_value()?))
 }
 
-#[cfg(test)]
+// these tests depend on the `typetag` crate.
+#[cfg(all(test, not(miri)))]
 mod test {
 
     use rstest::rstest;