diff --git a/common/calculate-cpmm.ts b/common/calculate-cpmm.ts
index d36d2f21..4c0d30eb 100644
--- a/common/calculate-cpmm.ts
+++ b/common/calculate-cpmm.ts
@@ -172,7 +172,7 @@ export function calculateCpmmSale(
   shares: number,
   outcome: string
 ) {
-  if (shares < 0) {
+  if (Math.round(shares) < 0) {
     throw new Error('Cannot sell non-positive shares')
   }