diff --git a/compilers/concrete-optimizer/concrete-optimizer/src/optimization/dag/solo_key/analyze.rs b/compilers/concrete-optimizer/concrete-optimizer/src/optimization/dag/solo_key/analyze.rs
index 2e86858e5..02f8e14c9 100644
--- a/compilers/concrete-optimizer/concrete-optimizer/src/optimization/dag/solo_key/analyze.rs
+++ b/compilers/concrete-optimizer/concrete-optimizer/src/optimization/dag/solo_key/analyze.rs
@@ -52,11 +52,25 @@ fn assert_non_empty_inputs(op: &unparametrized::UnparameterizedOperator) {
     }
 }
 
+fn assert_inputs_index(op: &unparametrized::UnparameterizedOperator, first_bad_index: usize) {
+    let valid = match op {
+        Op::Input { .. } => true,
+        Op::Lut { input, .. } | Op::UnsafeCast { input, .. } | Op::Round { input, .. } => {
+            input.i < first_bad_index
+        }
+        Op::LevelledOp { inputs, .. } | Op::Dot { inputs, .. } => {
+            inputs.iter().all(|input| input.i < first_bad_index)
+        }
+    };
+    assert!(valid, "Invalid dag, bad index in op: {op:?}");
+}
+
 fn assert_dag_correctness(dag: &unparametrized::OperationDag) {
-    for op in &dag.operators {
+    for (i, op) in dag.operators.iter().enumerate() {
         assert_non_empty_inputs(op);
         assert_inputs_uniform_precisions(op, &dag.out_precisions);
         assert_dot_uniform_inputs_shape(op, &dag.out_shapes);
+        assert_inputs_index(op, i);
     }
 }