diff --git a/src/backend/function-builder.js b/src/backend/function-builder.js
index 1633f0e8..4b61dc92 100644
--- a/src/backend/function-builder.js
+++ b/src/backend/function-builder.js
@@ -576,7 +576,7 @@ class FunctionBuilder {
       }
       return calleeBitRatio;
     }
-    calleeNode.argumentBitRatios[i] = bitRatio;
+    calleeNode.argumentBitRatios[argumentIndex] = bitRatio;
     return bitRatio;
   }
 
@@ -624,4 +624,4 @@ class FunctionBuilder {
 
 module.exports = {
   FunctionBuilder
-};
\ No newline at end of file
+};