diff --git a/src/fri/recursive_verifier.rs b/src/fri/recursive_verifier.rs
index fc90be35..35fd6765 100644
--- a/src/fri/recursive_verifier.rs
+++ b/src/fri/recursive_verifier.rs
@@ -210,8 +210,12 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
     ) -> ExtensionTarget<D> {
         assert!(D > 1, "Not implemented for D=1.");
         let config = self.config.clone();
-        let degree_log = common_data.config.cap_height + proof.evals_proofs[0].1.siblings.len()
-            - config.rate_bits;
+        let degree_log = common_data.degree_bits;
+        debug_assert_eq!(
+            degree_log,
+            common_data.config.cap_height + proof.evals_proofs[0].1.siblings.len()
+                - config.rate_bits
+        );
         let subgroup_x = self.convert_to_ext(subgroup_x);
         let vanish_zeta = self.sub_extension(subgroup_x, zeta);
         let mut alpha = ReducingFactorTarget::new(alpha);
@@ -292,7 +296,6 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         let mut x_index_bits = self.low_bits(x_index, n_log, 64);
         let cap_index = self.le_sum(
             x_index_bits[x_index_bits.len() - common_data.config.fri_config.cap_height..]
-                .to_vec()
                 .into_iter(),
         );
         let mut domain_size = n;
diff --git a/src/fri/verifier.rs b/src/fri/verifier.rs
index e9f64720..021ab122 100644
--- a/src/fri/verifier.rs
+++ b/src/fri/verifier.rs
@@ -189,8 +189,11 @@ fn fri_combine_initial<F: Field + Extendable<D>, const D: usize>(
 ) -> F::Extension {
     let config = &common_data.config;
     assert!(D > 1, "Not implemented for D=1.");
-    let degree_log =
-        common_data.config.cap_height + proof.evals_proofs[0].1.siblings.len() - config.rate_bits;
+    let degree_log = common_data.degree_bits;
+    debug_assert_eq!(
+        degree_log,
+        common_data.config.cap_height + proof.evals_proofs[0].1.siblings.len() - config.rate_bits
+    );
     let subgroup_x = F::Extension::from_basefield(subgroup_x);
     let mut alpha = ReducingFactor::new(alpha);
     let mut sum = F::Extension::ZERO;
diff --git a/src/hash/merkle_proofs.rs b/src/hash/merkle_proofs.rs
index 5b99fa19..45009c7d 100644
--- a/src/hash/merkle_proofs.rs
+++ b/src/hash/merkle_proofs.rs
@@ -1,3 +1,5 @@
+use std::convert::TryInto;
+
 use anyhow::{ensure, Result};
 use serde::{Deserialize, Serialize};
 
@@ -111,15 +113,13 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
         }
 
         let index = self.le_sum(leaf_index_bits[proof.siblings.len()..].to_vec().into_iter());
-        let mut state_ext = [zero; D];
-        state_ext[..D].copy_from_slice(&state.elements[..D]);
+        let state_ext = state.elements[..].try_into().expect("requires D = 4");
         let state_ext = ExtensionTarget(state_ext);
         let cap_ext = merkle_cap
             .0
             .iter()
             .map(|h| {
-                let mut tmp = [zero; D];
-                tmp[..D].copy_from_slice(&h.elements[..D]);
+                let tmp = h.elements[..].try_into().expect("requires D = 4");
                 ExtensionTarget(tmp)
             })
             .collect();
@@ -178,15 +178,13 @@ impl<F: Extendable<D>, const D: usize> CircuitBuilder<F, D> {
             )
         }
 
-        let mut state_ext = [zero; D];
-        state_ext[..D].copy_from_slice(&state.elements[..D]);
+        let state_ext = state.elements[..].try_into().expect("requires D = 4");
         let state_ext = ExtensionTarget(state_ext);
         let cap_ext = merkle_cap
             .0
             .iter()
             .map(|h| {
-                let mut tmp = [zero; D];
-                tmp[..D].copy_from_slice(&h.elements[..D]);
+                let tmp = h.elements[..].try_into().expect("requires D = 4");
                 ExtensionTarget(tmp)
             })
             .collect();
diff --git a/src/hash/merkle_tree.rs b/src/hash/merkle_tree.rs
index 08e49c42..334e65bf 100644
--- a/src/hash/merkle_tree.rs
+++ b/src/hash/merkle_tree.rs
@@ -7,6 +7,8 @@ use crate::hash::hashing::{compress, hash_or_noop};
 use crate::hash::merkle_proofs::MerkleProof;
 use crate::util::{log2_strict, reverse_bits, reverse_index_bits_in_place};
 
+/// The Merkle cap of height `h` of a Merkle tree is the `h`-th layer (from the root) of the tree.
+/// It can be used in place of the root to verify Merkle paths, which are `h` elements shorter.
 #[derive(Clone, Debug, Serialize, Deserialize)]
 #[serde(bound = "")]
 pub struct MerkleCap<F: Field>(pub Vec<HashOut<F>>);