diff --git a/gext/fitting.py b/gext/fitting.py
index 0301c05c3ce3cfd311e7f1b58c23954a543352c3..dbc5a50f271b43f071d54ae06881c102c77087dd 100644
--- a/gext/fitting.py
+++ b/gext/fitting.py
@@ -65,8 +65,8 @@ class DiffFitting(AbstractFitting):
             raise ValueError("DiffFit does not work for one vector")
         target=target-vectors[-1]
         VECTORS=[]
-        for i in range(0, len(vectors)+1):
-            VECTORS.append(vectors[i]-vectors[-1])
+        for i in range(1, len(vectors)):
+            VECTORS.append(vectors[i-1]-vectors[-1])
         matrix = np.array(VECTORS).T
         a = matrix.T @ matrix
         b = matrix.T @ target
@@ -84,8 +84,8 @@ class DiffFitting(AbstractFitting):
             raise ValueError("DiffFit does not work for one vector")
         result = np.zeros(vectors[0].shape, dtype=np.float64)
         VECTORS_DiffFitting=[]
-        for i in range(0,len(vectors)+1):
-              VECTORS_DiffFitting.append(vectors[i]-vectors[-1])
+        for i in range(1,len(vectors)):
+              VECTORS_DiffFitting.append(vectors[i-1]-vectors[-1])
         for coeff, vector in zip(coefficients, VECTORS_DiffFitting):
             result += vector*coeff
         result=result+vectors[-1]