diff --git a/hugr-core/src/ops/constant/custom.rs b/hugr-core/src/ops/constant/custom.rs
index eb46a2bb7..e3df87b45 100644
--- a/hugr-core/src/ops/constant/custom.rs
+++ b/hugr-core/src/ops/constant/custom.rs
@@ -476,44 +476,45 @@ mod test {
             serde_yaml::from_value(serde_yaml::to_value(&ev).unwrap()).unwrap()
         );
     }
+}
 
-    mod proptest {
-        use ::proptest::prelude::*;
-
-        use crate::{
-            extension::ExtensionSet,
-            ops::constant::CustomSerialized,
-            proptest::{any_serde_yaml_value, any_string},
-            types::Type,
-        };
-
-        impl Arbitrary for CustomSerialized {
-            type Parameters = ();
-            type Strategy = BoxedStrategy<Self>;
-            fn arbitrary_with(_args: Self::Parameters) -> Self::Strategy {
-                let typ = any::<Type>();
-                let extensions = any::<ExtensionSet>();
-                // here we manually construct a serialized `dyn CustomConst`.
-                // The "c" and "v" come from the `typetag::serde` annotation on
-                // `trait CustomConst`.
-                // TODO This is not ideal, if we were to accidentally
-                // generate a valid tag(e.g. "ConstInt") then things will
-                // go wrong: the serde::Deserialize impl for that type will
-                // interpret "v" and fail.
-                let value = (any_serde_yaml_value(), any_string()).prop_map(|(content, tag)| {
-                    [("c".into(), tag.into()), ("v".into(), content)]
-                        .into_iter()
-                        .collect::<serde_yaml::Mapping>()
-                        .into()
-                });
-                (typ, value, extensions)
-                    .prop_map(|(typ, value, extensions)| CustomSerialized {
-                        typ,
-                        value,
-                        extensions,
-                    })
-                    .boxed()
-            }
+#[cfg(test)]
+mod proptest {
+    use ::proptest::prelude::*;
+
+    use crate::{
+        extension::ExtensionSet,
+        ops::constant::CustomSerialized,
+        proptest::{any_serde_yaml_value, any_string},
+        types::Type,
+    };
+
+    impl Arbitrary for CustomSerialized {
+        type Parameters = ();
+        type Strategy = BoxedStrategy<Self>;
+        fn arbitrary_with(_args: Self::Parameters) -> Self::Strategy {
+            let typ = any::<Type>();
+            let extensions = any::<ExtensionSet>();
+            // here we manually construct a serialized `dyn CustomConst`.
+            // The "c" and "v" come from the `typetag::serde` annotation on
+            // `trait CustomConst`.
+            // TODO This is not ideal, if we were to accidentally
+            // generate a valid tag(e.g. "ConstInt") then things will
+            // go wrong: the serde::Deserialize impl for that type will
+            // interpret "v" and fail.
+            let value = (any_serde_yaml_value(), any_string()).prop_map(|(content, tag)| {
+                [("c".into(), tag.into()), ("v".into(), content)]
+                    .into_iter()
+                    .collect::<serde_yaml::Mapping>()
+                    .into()
+            });
+            (typ, value, extensions)
+                .prop_map(|(typ, value, extensions)| CustomSerialized {
+                    typ,
+                    value,
+                    extensions,
+                })
+                .boxed()
         }
     }
 }