diff --git a/yarn-project/foundation/src/abi/u128.ts b/yarn-project/foundation/src/abi/u128.ts
index 52a157e09698..14fda5848955 100644
--- a/yarn-project/foundation/src/abi/u128.ts
+++ b/yarn-project/foundation/src/abi/u128.ts
@@ -5,7 +5,7 @@ export class U128 {
   private readonly value: bigint;
 
   constructor(value: bigint | number) {
-    if (typeof value === 'number') {
+    if (typeof value !== 'bigint') {
       value = BigInt(value);
     }