diff --git a/eth2/utils/cached_tree_hash/src/btree_overlay.rs b/eth2/utils/cached_tree_hash/src/btree_overlay.rs
index 1a8fde3c1..866f5c6b7 100644
--- a/eth2/utils/cached_tree_hash/src/btree_overlay.rs
+++ b/eth2/utils/cached_tree_hash/src/btree_overlay.rs
@@ -4,33 +4,22 @@ use super::*;
 pub struct BTreeOverlay {
     pub offset: usize,
     pub depth: usize,
-    pub num_items: usize,
     pub lengths: Vec<usize>,
 }
 
 impl BTreeOverlay {
-    pub fn new<T>(item: &T, initial_offset: usize, depth: usize) -> Result<Self, Error>
+    pub fn new<T>(item: &T, initial_offset: usize, depth: usize) -> Self
     where
         T: CachedTreeHash<T>,
     {
         item.tree_hash_cache_overlay(initial_offset, depth)
     }
 
-    pub fn from_lengths(
-        offset: usize,
-        num_items: usize,
-        depth: usize,
-        lengths: Vec<usize>,
-    ) -> Result<Self, Error> {
-        if lengths.is_empty() {
-            Err(Error::TreeCannotHaveZeroNodes)
-        } else {
-            Ok(Self {
-                offset,
-                num_items,
-                depth,
-                lengths,
-            })
+    pub fn from_lengths(offset: usize, depth: usize, lengths: Vec<usize>) -> Self {
+        Self {
+            offset,
+            depth,
+            lengths,
         }
     }
 
@@ -96,7 +85,7 @@ impl BTreeOverlay {
     pub fn get_leaf_node(&self, i: usize) -> Result<Option<Range<usize>>, Error> {
         if i >= self.num_nodes() - self.num_padding_leaves() {
             Ok(None)
-        } else if (i == self.num_internal_nodes()) && (self.num_items == 0) {
+        } else if (i == self.num_internal_nodes()) && (self.lengths.len() == 0) {
             // If this is the first leaf node and the overlay contains zero items, return `None` as
             // this node must be padding.
             Ok(None)
@@ -177,7 +166,7 @@ mod test {
     use super::*;
 
     fn get_tree_a(n: usize) -> BTreeOverlay {
-        BTreeOverlay::from_lengths(0, n, 0, vec![1; n]).unwrap()
+        BTreeOverlay::from_lengths(0, 0, vec![1; n])
     }
 
     #[test]
@@ -215,7 +204,7 @@ mod test {
         let tree = get_tree_a(2);
         assert_eq!(tree.chunk_range(), 0..3);
 
-        let tree = BTreeOverlay::from_lengths(11, 4, 0, vec![1, 1]).unwrap();
+        let tree = BTreeOverlay::from_lengths(11, 0, vec![1, 1]);
         assert_eq!(tree.chunk_range(), 11..14);
     }
 
diff --git a/eth2/utils/cached_tree_hash/src/impls.rs b/eth2/utils/cached_tree_hash/src/impls.rs
index 827b0fbe9..666461962 100644
--- a/eth2/utils/cached_tree_hash/src/impls.rs
+++ b/eth2/utils/cached_tree_hash/src/impls.rs
@@ -16,13 +16,8 @@ impl CachedTreeHash<u64> for u64 {
         1
     }
 
-    fn tree_hash_cache_overlay(
-        &self,
-        _chunk_offset: usize,
-        _depth: usize,
-    ) -> Result<BTreeOverlay, Error> {
-        panic!("Basic should not produce overlay");
-        // BTreeOverlay::from_lengths(chunk_offset, 1, depth, vec![1])
+    fn tree_hash_cache_overlay(&self, _chunk_offset: usize, _depth: usize) -> BTreeOverlay {
+        unreachable!("Basic should not produce overlay");
     }
 
     fn update_tree_hash_cache(&self, cache: &mut TreeHashCache) -> Result<(), Error> {
@@ -49,13 +44,8 @@ impl CachedTreeHash<usize> for usize {
         1
     }
 
-    fn tree_hash_cache_overlay(
-        &self,
-        _chunk_offset: usize,
-        _depth: usize,
-    ) -> Result<BTreeOverlay, Error> {
-        panic!("Basic should not produce overlay");
-        // BTreeOverlay::from_lengths(chunk_offset, 1, depth, vec![1])
+    fn tree_hash_cache_overlay(&self, _chunk_offset: usize, _depth: usize) -> BTreeOverlay {
+        unreachable!("Basic should not produce overlay");
     }
 
     fn update_tree_hash_cache(&self, cache: &mut TreeHashCache) -> Result<(), Error> {
diff --git a/eth2/utils/cached_tree_hash/src/impls/vec.rs b/eth2/utils/cached_tree_hash/src/impls/vec.rs
index 14802254b..c37354404 100644
--- a/eth2/utils/cached_tree_hash/src/impls/vec.rs
+++ b/eth2/utils/cached_tree_hash/src/impls/vec.rs
@@ -14,17 +14,11 @@ where
     }
 
     fn num_tree_hash_cache_chunks(&self) -> usize {
-        BTreeOverlay::new(self, 0, 0)
-            .and_then(|o| Ok(o.num_chunks()))
-            .unwrap_or_else(|_| 1)
-            + 2 // Add two extra nodes to cater for the length.
+        // Add two extra nodes to cater for the node before and after to allow mixing-in length.
+        BTreeOverlay::new(self, 0, 0).num_chunks() + 2
     }
 
-    fn tree_hash_cache_overlay(
-        &self,
-        chunk_offset: usize,
-        depth: usize,
-    ) -> Result<BTreeOverlay, Error> {
+    fn tree_hash_cache_overlay(&self, chunk_offset: usize, depth: usize) -> BTreeOverlay {
         produce_overlay(self, chunk_offset, depth)
     }
 
@@ -49,7 +43,7 @@ pub fn new_tree_hash_cache<T: CachedTreeHash<T>>(
     vec: &Vec<T>,
     depth: usize,
 ) -> Result<(TreeHashCache, BTreeOverlay), Error> {
-    let overlay = vec.tree_hash_cache_overlay(0, depth)?;
+    let overlay = vec.tree_hash_cache_overlay(0, depth);
 
     let cache = match T::tree_hash_type() {
         TreeHashType::Basic => TreeHashCache::from_bytes(
@@ -74,7 +68,7 @@ pub fn produce_overlay<T: CachedTreeHash<T>>(
     vec: &Vec<T>,
     chunk_offset: usize,
     depth: usize,
-) -> Result<BTreeOverlay, Error> {
+) -> BTreeOverlay {
     let lengths = match T::tree_hash_type() {
         TreeHashType::Basic => {
             // Ceil division.
@@ -91,16 +85,11 @@ pub fn produce_overlay<T: CachedTreeHash<T>>(
                 lengths.push(item.num_tree_hash_cache_chunks())
             }
 
-            // Disallow zero-length as an empty list still has one all-padding node.
-            if lengths.is_empty() {
-                lengths.push(1);
-            }
-
             lengths
         }
     };
 
-    BTreeOverlay::from_lengths(chunk_offset, vec.len(), depth, lengths)
+    BTreeOverlay::from_lengths(chunk_offset, depth, lengths)
 }
 
 pub fn update_tree_hash_cache<T: CachedTreeHash<T>>(
@@ -108,7 +97,7 @@ pub fn update_tree_hash_cache<T: CachedTreeHash<T>>(
     cache: &mut TreeHashCache,
 ) -> Result<BTreeOverlay, Error> {
     let old_overlay = cache.get_overlay(cache.overlay_index, cache.chunk_index)?;
-    let new_overlay = BTreeOverlay::new(vec, cache.chunk_index, old_overlay.depth)?;
+    let new_overlay = BTreeOverlay::new(vec, cache.chunk_index, old_overlay.depth);
 
     cache.replace_overlay(cache.overlay_index, cache.chunk_index, new_overlay.clone())?;
 
@@ -183,7 +172,7 @@ pub fn update_tree_hash_cache<T: CachedTreeHash<T>>(
                     //
                     // Viz., the list has been shortened.
                     (Some(old), None) => {
-                        if new_overlay.num_items == 0 {
+                        if vec.len() == 0 {
                             // In this case, the list has been made empty and we should make
                             // this node padding.
                             cache.maybe_update_chunk(new_overlay.root(), &[0; HASHSIZE])?;
diff --git a/eth2/utils/cached_tree_hash/src/lib.rs b/eth2/utils/cached_tree_hash/src/lib.rs
index d9f2b6a84..2cf3c38bd 100644
--- a/eth2/utils/cached_tree_hash/src/lib.rs
+++ b/eth2/utils/cached_tree_hash/src/lib.rs
@@ -14,11 +14,7 @@ pub use errors::Error;
 pub use tree_hash_cache::TreeHashCache;
 
 pub trait CachedTreeHash<Item>: TreeHash {
-    fn tree_hash_cache_overlay(
-        &self,
-        chunk_offset: usize,
-        depth: usize,
-    ) -> Result<BTreeOverlay, Error>;
+    fn tree_hash_cache_overlay(&self, chunk_offset: usize, depth: usize) -> BTreeOverlay;
 
     fn num_tree_hash_cache_chunks(&self) -> usize;
 
diff --git a/eth2/utils/cached_tree_hash/src/tree_hash_cache.rs b/eth2/utils/cached_tree_hash/src/tree_hash_cache.rs
index d93278d30..2a9bae5a3 100644
--- a/eth2/utils/cached_tree_hash/src/tree_hash_cache.rs
+++ b/eth2/utils/cached_tree_hash/src/tree_hash_cache.rs
@@ -34,7 +34,7 @@ impl TreeHashCache {
     where
         T: CachedTreeHash<T>,
     {
-        let overlay = BTreeOverlay::new(item, 0, depth)?;
+        let overlay = BTreeOverlay::new(item, 0, depth);
 
         // Note how many leaves were provided. If is not a power-of-two, we'll need to pad it out
         // later.
diff --git a/eth2/utils/tree_hash_derive/src/lib.rs b/eth2/utils/tree_hash_derive/src/lib.rs
index b6ba10e02..104cb8b32 100644
--- a/eth2/utils/tree_hash_derive/src/lib.rs
+++ b/eth2/utils/tree_hash_derive/src/lib.rs
@@ -55,8 +55,6 @@ pub fn subtree_derive(input: TokenStream) -> TokenStream {
     let idents_b = idents_a.clone();
     let idents_c = idents_a.clone();
 
-    let num_items = idents_a.len();
-
     let output = quote! {
         impl cached_tree_hash::CachedTreeHash<#name> for #name {
             fn new_tree_hash_cache(&self, depth: usize) -> Result<cached_tree_hash::TreeHashCache, cached_tree_hash::Error> {
@@ -74,23 +72,21 @@ pub fn subtree_derive(input: TokenStream) -> TokenStream {
             }
 
             fn num_tree_hash_cache_chunks(&self) -> usize {
-                cached_tree_hash::BTreeOverlay::new(self, 0, 0)
-                    .and_then(|o| Ok(o.num_chunks()))
-                    .unwrap_or_else(|_| 1)
+                cached_tree_hash::BTreeOverlay::new(self, 0, 0).num_chunks()
             }
 
-            fn tree_hash_cache_overlay(&self, chunk_offset: usize, depth: usize) -> Result<cached_tree_hash::BTreeOverlay, cached_tree_hash::Error> {
+            fn tree_hash_cache_overlay(&self, chunk_offset: usize, depth: usize) ->cached_tree_hash::BTreeOverlay {
                 let mut lengths = vec![];
 
                 #(
                     lengths.push(self.#idents_b.num_tree_hash_cache_chunks());
                 )*
 
-                cached_tree_hash::BTreeOverlay::from_lengths(chunk_offset, #num_items, depth, lengths)
+                cached_tree_hash::BTreeOverlay::from_lengths(chunk_offset, depth, lengths)
             }
 
             fn update_tree_hash_cache(&self, cache: &mut cached_tree_hash::TreeHashCache) -> Result<(), cached_tree_hash::Error> {
-                let overlay = cached_tree_hash::BTreeOverlay::new(self, cache.chunk_index, 0)?;
+                let overlay = cached_tree_hash::BTreeOverlay::new(self, cache.chunk_index, 0);
 
                 // Skip the chunk index to the first leaf node of this struct.
                 cache.chunk_index = overlay.first_leaf_node();