diff --git a/zebra-chain/src/sapling/tree.rs b/zebra-chain/src/sapling/tree.rs
index 0339a7a38..6e29849e0 100644
--- a/zebra-chain/src/sapling/tree.rs
+++ b/zebra-chain/src/sapling/tree.rs
@@ -158,6 +158,7 @@ mod tests {
 
     use super::*;
 
+    // From https://github.com/zcash/librustzcash/blob/master/zcash_primitives/src/merkle_tree.rs#L512
     const HEX_EMPTY_ROOTS: [&str; 33] = [
         "0100000000000000000000000000000000000000000000000000000000000000",
         "817de36ab2d57feb077634bca77819c8e0bd298c04f6fed0e6a83cc1356ca155",