diff --git a/lib/bill-math.js b/lib/bill-math.js
index b53742c3..fa0b8db8 100644
--- a/lib/bill-math.js
+++ b/lib/bill-math.js
@@ -155,7 +155,7 @@ function makeChange(outCassettes, amount) {
   const ss_solution = getSolution_old(outCassettes, amount, BILL_LIST_MODES.VALUE_ROUND_ROBIN)
   const cc_solution = getSolution(outCassettes, amount)
 
-  if (!!ss_solution === !!cc_solution) {
+  if (!!ss_solution !== !!cc_solution) {
     logger.error(new Error(`subset-sum and coin-change don't agree on solvability -- subset-sum:${!!ss_solution} coin-change:${!!cc_solution}`))
     return solutionToOriginalUnits(ss_solution, outCassettes)
   }