diff --git a/compilers/concrete-optimizer/concrete-optimizer/src/optimization/dag/multi_parameters/optimize.rs b/compilers/concrete-optimizer/concrete-optimizer/src/optimization/dag/multi_parameters/optimize.rs
index 7d515aef4..7152f0d81 100644
--- a/compilers/concrete-optimizer/concrete-optimizer/src/optimization/dag/multi_parameters/optimize.rs
+++ b/compilers/concrete-optimizer/concrete-optimizer/src/optimization/dag/multi_parameters/optimize.rs
@@ -750,10 +750,19 @@ fn optimize_macro(
                 // copy back pbs from other partition
                 let mut all_pbs = init_parameters.micro_params.pbs.clone();
                 all_pbs[partition] = Some(some_micro_params.pbs);
+                let mut all_fks = init_parameters.micro_params.fks.clone();
+                for (dst_partition, maybe_fks) in fks_to_optimize.iter().enumerate() {
+                    if let &Some(src_partition) = maybe_fks {
+                        all_fks[src_partition][dst_partition] =
+                            some_micro_params.fks[src_partition][dst_partition];
+                        assert!(used_conversion_keyswitch[src_partition][dst_partition]);
+                        assert!(all_fks[src_partition][dst_partition].is_some());
+                    }
+                }
                 let micro_params = MicroParameters {
                     pbs: all_pbs,
                     ks: some_micro_params.ks,
-                    fks: some_micro_params.fks,
+                    fks: all_fks,
                 };
                 // for (i, pbs) in init_parameters.micro_params.pbs.iter().enumerate() {
                 //     if i != partition {
@@ -900,6 +909,8 @@ pub fn optimize(
     }
     sanity_check(
         &params,
+        &used_conversion_keyswitch,
+        &used_tlu_keyswitch,
         ciphertext_modulus_log,
         security_level,
         &feasible,
@@ -945,6 +956,8 @@ fn used_conversion_keyswitch(dag: &AnalyzedDag) -> Vec<Vec<bool>> {
 #[allow(clippy::float_cmp)]
 fn sanity_check(
     params: &Parameters,
+    used_conversion_keyswitch: &[Vec<bool>],
+    used_tlu_keyswitch: &[Vec<bool>],
     ciphertext_modulus_log: u32,
     security_level: u64,
     feasible: &Feasible,
@@ -980,16 +993,32 @@ fn sanity_check(
             let src_glwe_param = src_partition_macro.glwe_params;
             let src_lwe_dim = src_glwe_param.sample_extract_lwe_dimension();
             if let Some(ks) = micro_params.ks[src_partition][partition] {
+                assert!(
+                    used_tlu_keyswitch[src_partition][partition],
+                    "Superflous ks[{src_partition}->{partition}]"
+                );
                 *operations.variance.ks(src_partition, partition) = ks.noise(src_lwe_dim);
                 *operations.cost.ks(src_partition, partition) = ks.complexity(src_lwe_dim);
             } else {
+                assert!(
+                    !used_tlu_keyswitch[src_partition][partition],
+                    "Missing ks[{src_partition}->{partition}]"
+                );
                 *operations.variance.ks(src_partition, partition) = f64::MAX;
                 *operations.cost.ks(src_partition, partition) = f64::MAX;
             }
             if let Some(fks) = micro_params.fks[src_partition][partition] {
+                assert!(
+                    used_conversion_keyswitch[src_partition][partition],
+                    "Superflous fks[{src_partition}->{partition}]"
+                );
                 *operations.variance.fks(src_partition, partition) = fks.noise;
                 *operations.cost.fks(src_partition, partition) = fks.complexity;
             } else {
+                assert!(
+                    !used_conversion_keyswitch[src_partition][partition],
+                    "Missing fks[{src_partition}->{partition}]"
+                );
                 *operations.variance.fks(src_partition, partition) = f64::MAX;
                 *operations.cost.fks(src_partition, partition) = f64::MAX;
             }