diff --git a/hugr-core/src/hugr/views/sibling_subgraph.rs b/hugr-core/src/hugr/views/sibling_subgraph.rs
index ddc307509..a024bcc17 100644
--- a/hugr-core/src/hugr/views/sibling_subgraph.rs
+++ b/hugr-core/src/hugr/views/sibling_subgraph.rs
@@ -345,7 +345,12 @@ impl SiblingSubgraph {
         let Some([rep_input, rep_output]) = replacement.get_io(rep_root) else {
             return Err(InvalidReplacement::InvalidDataflowParent);
         };
-        if dfg_optype.dataflow_signature() != Some(self.signature(hugr)) {
+
+        let current_signature = self.signature(hugr);
+        let new_signature = dfg_optype.dataflow_signature();
+        if new_signature.as_ref().map(|s| &s.input) != Some(&current_signature.input)
+            || new_signature.as_ref().map(|s| &s.output) != Some(&current_signature.output)
+        {
             return Err(InvalidReplacement::InvalidSignature);
         }