diff --git a/eth2/utils/ssz/src/cached_tree_hash.rs b/eth2/utils/ssz/src/cached_tree_hash.rs
index a85da8fd9..757bfa9f7 100644
--- a/eth2/utils/ssz/src/cached_tree_hash.rs
+++ b/eth2/utils/ssz/src/cached_tree_hash.rs
@@ -10,6 +10,7 @@ pub struct TreeHashCache<'a> {
     chunk_offset: usize,
     cache: &'a mut [u8],
     chunk_modified: &'a mut [bool],
+    hash_count: &'a mut usize,
 }
 
 impl<'a> TreeHashCache<'a> {
@@ -17,17 +18,20 @@ impl<'a> TreeHashCache<'a> {
         vec![false; bytes.len() / BYTES_PER_CHUNK]
     }
 
-    pub fn from_mut_slice(bytes: &'a mut [u8], changes: &'a mut [bool]) -> Option<Self> {
+    pub fn from_mut_slice(
+        bytes: &'a mut [u8],
+        changes: &'a mut [bool],
+        hash_count: &'a mut usize,
+    ) -> Option<Self> {
         if bytes.len() % BYTES_PER_CHUNK > 0 {
             return None;
         }
 
-        let chunk_modified = vec![false; bytes.len() / BYTES_PER_CHUNK];
-
         Some(Self {
             chunk_offset: 0,
             cache: bytes,
             chunk_modified: changes,
+            hash_count,
         })
     }
 
@@ -36,12 +40,13 @@ impl<'a> TreeHashCache<'a> {
     }
 
     pub fn modify_current_chunk(&mut self, to: &[u8]) -> Option<()> {
-        self.modify_chunk(0, to)
+        self.modify_chunk(self.chunk_offset, to)
     }
 
     pub fn modify_chunk(&mut self, chunk: usize, to: &[u8]) -> Option<()> {
         let start = chunk * BYTES_PER_CHUNK;
         let end = start + BYTES_PER_CHUNK;
+
         self.cache.get_mut(start..end)?.copy_from_slice(to);
 
         self.chunk_modified[chunk] = true;
@@ -79,9 +84,10 @@ impl<'a> TreeHashCache<'a> {
         let modified_end = modified_start + leaves;
 
         Some(TreeHashCache {
-            chunk_offset: self.chunk_offset + internal,
+            chunk_offset: 0,
             cache: self.cache.get_mut(leaves_start..leaves_end)?,
             chunk_modified: self.chunk_modified.get_mut(modified_start..modified_end)?,
+            hash_count: self.hash_count,
         })
     }
 
@@ -111,7 +117,8 @@ impl CachedTreeHash for u64 {
 
     fn cached_hash_tree_root(&self, other: &Self, cache: &mut TreeHashCache) -> Option<()> {
         if self != other {
-            cache.modify_current_chunk(&merkleize(&int_to_bytes32(*self)));
+            *cache.hash_count += 1;
+            cache.modify_current_chunk(&merkleize(&int_to_bytes32(*self)))?;
         }
 
         cache.increment();
@@ -156,6 +163,7 @@ impl CachedTreeHash for Inner {
 
         for chunk in (0..internal_chunks).into_iter().rev() {
             if cache.children_modified(chunk)? {
+                *cache.hash_count += 1;
                 cache.modify_chunk(chunk, &cache.hash_children(chunk)?)?;
             }
         }
@@ -197,7 +205,7 @@ pub fn merkleize(values: &[u8]) -> Vec<u8> {
 mod tests {
     use super::*;
 
-    fn join(many: Vec<&[u8]>) -> Vec<u8> {
+    fn join(many: Vec<Vec<u8>>) -> Vec<u8> {
         let mut all = vec![];
         for one in many {
             all.extend_from_slice(&mut one.clone())
@@ -205,8 +213,7 @@ mod tests {
         all
     }
 
-    #[test]
-    fn cached_hash_on_inner() {
+    fn generic_test(index: usize) {
         let inner = Inner {
             a: 1,
             b: 2,
@@ -216,37 +223,69 @@ mod tests {
 
         let mut cache = inner.build_cache_bytes();
 
-        let changed_inner = Inner {
-            a: 42,
-            ..inner.clone()
+        let changed_inner = match index {
+            0 => Inner {
+                a: 42,
+                ..inner.clone()
+            },
+            1 => Inner {
+                b: 42,
+                ..inner.clone()
+            },
+            2 => Inner {
+                c: 42,
+                ..inner.clone()
+            },
+            3 => Inner {
+                d: 42,
+                ..inner.clone()
+            },
+            _ => panic!("bad index"),
         };
 
         let mut changes = TreeHashCache::build_changes_vec(&cache);
-        let mut cache_struct = TreeHashCache::from_mut_slice(&mut cache, &mut changes).unwrap();
+        let mut hash_count = 0;
+        let mut cache_struct =
+            TreeHashCache::from_mut_slice(&mut cache, &mut changes, &mut hash_count).unwrap();
 
-        changed_inner.cached_hash_tree_root(&inner, &mut cache_struct);
+        changed_inner
+            .cached_hash_tree_root(&inner, &mut cache_struct)
+            .unwrap();
+
+        assert_eq!(*cache_struct.hash_count, 3);
 
         let new_cache = cache_struct.into_slice();
 
-        let data1 = &int_to_bytes32(42);
-        let data2 = &int_to_bytes32(2);
-        let data3 = &int_to_bytes32(3);
-        let data4 = &int_to_bytes32(4);
+        let data1 = int_to_bytes32(1);
+        let data2 = int_to_bytes32(2);
+        let data3 = int_to_bytes32(3);
+        let data4 = int_to_bytes32(4);
 
-        let data = join(vec![&data1, &data2, &data3, &data4]);
-        let expected = merkleize(&data);
+        let mut data = vec![data1, data2, data3, data4];
+
+        data[index] = int_to_bytes32(42);
+
+        let expected = merkleize(&join(data));
 
         assert_eq!(expected, new_cache);
     }
 
     #[test]
-    fn build_cache_matches_merkelize() {
-        let data1 = &int_to_bytes32(1);
-        let data2 = &int_to_bytes32(2);
-        let data3 = &int_to_bytes32(3);
-        let data4 = &int_to_bytes32(4);
+    fn cached_hash_on_inner() {
+        generic_test(0);
+        generic_test(1);
+        generic_test(2);
+        generic_test(3);
+    }
 
-        let data = join(vec![&data1, &data2, &data3, &data4]);
+    #[test]
+    fn build_cache_matches_merkelize() {
+        let data1 = int_to_bytes32(1);
+        let data2 = int_to_bytes32(2);
+        let data3 = int_to_bytes32(3);
+        let data4 = int_to_bytes32(4);
+
+        let data = join(vec![data1, data2, data3, data4]);
         let expected = merkleize(&data);
 
         let inner = Inner {
@@ -268,7 +307,12 @@ mod tests {
         let data3 = hash(&int_to_bytes32(3));
         let data4 = hash(&int_to_bytes32(4));
 
-        let data = join(vec![&data1, &data2, &data3, &data4]);
+        let data = join(vec![
+            data1.clone(),
+            data2.clone(),
+            data3.clone(),
+            data4.clone(),
+        ]);
 
         let cache = merkleize(&data);