diff --git a/src/main/java/org/teacon/signin/client/PolynomialMapping.java b/src/main/java/org/teacon/signin/client/PolynomialMapping.java
index 29f4979..efb37ec 100644
--- a/src/main/java/org/teacon/signin/client/PolynomialMapping.java
+++ b/src/main/java/org/teacon/signin/client/PolynomialMapping.java
@@ -118,7 +118,7 @@ public void interpolate(double[] inputX, double[] inputY, double[] inputDX, doub
             Complex currentValue = current.sub(this.inputParameters.get(0)), resultValue = currentOutputFactor;
 
             for (int j = 1; j < degree; ++j) {
-                Complex inputFactor = current.sub(this.inputParameters.get(i));
+                Complex inputFactor = current.sub(this.inputParameters.get(j));
                 currentOutputFactor = currentOutput.next();
 
                 resultDerivative = resultDerivative.add(currentDerivative.mul(currentOutputFactor));