diff --git a/mnist_test.go b/mnist_test.go
index 5faf456..9e6a287 100644
--- a/mnist_test.go
+++ b/mnist_test.go
@@ -47,3 +47,16 @@ func TestLoad(t *testing.T) {
 	}
 	println(train.Count(), test.Count())
 }
+
+func TestSweeperNext(t *testing.T) {
+	train, _, err := Load("./data")
+	if err != nil {
+		t.Fatalf("load (%s)", err)
+	}
+	sweeper := train.Sweep()
+	var currentIndex = sweeper.i
+	sweeper.Next()
+	if currentIndex == sweeper.i {
+		t.Errorf("Next does not increase index")
+	}
+}
diff --git a/util.go b/util.go
index 1a015ed..1af888d 100644
--- a/util.go
+++ b/util.go
@@ -57,10 +57,12 @@ type Sweeper struct {
 // Next returns the next image and its label in the data set.
 // If the end is reached, present is set to false.
 func (sw *Sweeper) Next() (image RawImage, label Label, present bool) {
-	if sw.i >= len(sw.set.Images) {
+	var prevIndex = sw.i
+	sw.i += 1
+	if prevIndex >= len(sw.set.Images) {
 		return nil, 0, false
 	}
-	return sw.set.Images[sw.i], sw.set.Labels[sw.i], true
+	return sw.set.Images[prevIndex], sw.set.Labels[prevIndex], true
 }
 
 // Sweep creates a new sweep iterator over the data set