diff --git a/build/instructions_template.rs b/build/instructions_template.rs
index 7914964f..86cde2f3 100644
--- a/build/instructions_template.rs
+++ b/build/instructions_template.rs
@@ -1017,6 +1017,13 @@ fn generate_instruction_preface() -> TokenStream {
                     },
                 }
             }
+
+            pub fn is_external(&self) -> bool {
+                matches!(
+                    self,
+                    IndexingCodePtr::External(_) | IndexingCodePtr::DynamicExternal(_)
+                )
+            }
         }
 
         impl IndexingInstruction {
diff --git a/src/indexing.rs b/src/indexing.rs
index 4eb39e2d..2df05530 100644
--- a/src/indexing.rs
+++ b/src/indexing.rs
@@ -114,13 +114,13 @@ impl<'a> IndexingCodeMergingPtr<'a> {
             Some(OptArgIndexKey::Literal(_, _, constant, _)) => {
                 constants.insert(*constant, constant_ptr);
             }
+            _ if constant_ptr.is_external() => {
+                // this must be a defunct clause, because it's been deleted
+                // from the skeleton.
+                debug_assert!(constant_key.is_none());
+            }
             _ => {
-                if let IndexingCodePtr::DynamicExternal(_) = constant_ptr {
-                    // this must be a defunct clause, because it's been deleted
-                    // from the skeleton.
-                } else {
-                    unreachable!()
-                }
+                unreachable!()
             }
         }