diff --git a/lib/bill-math.js b/lib/bill-math.js
index d4801c0f..fbb3ee99 100644
--- a/lib/bill-math.js
+++ b/lib/bill-math.js
@@ -73,8 +73,9 @@ function unmergeCassettes(cassettes, output) {
 }
 
 function makeChangeDuo(cassettes, amount) {
-  const small = cassettes[0]
-  const large = cassettes[1]
+  // Initialize empty cassettes in case of undefined, due to same denomination across all cassettes results in a single merged cassette
+  const small = cassettes[0] ?? { denomination: 0, count: 0 }
+  const large = cassettes[1] ?? { denomination: 0, count: 0 }
 
   const largeDenom = large.denomination
   const smallDenom = small.denomination