diff --git a/bcutils.js b/bcutils.js
index 1d598bc..e55c990 100644
--- a/bcutils.js
+++ b/bcutils.js
@@ -97,13 +97,13 @@ function isValidTransId(transId) {
 }
 
 function extractParamsFromTransId(transId) {
-  if (isValidTransId(transId)) {
+  if (!isValidTransId(transId)) {
     return null;
   }
   const transIdStr = transId.toString();
   const params = [];
   for (let i = 0; i < 3; ++i) {
-    const param = transId.substr(transIdStr.length - 20 * i , 20);
+    const param = transId.substr(transIdStr.length - 20 * (3 - i), 20);
     params.push(utils.removeLeadZero(param));
   }
   return params;