diff --git a/internal/testutil/testutil.go b/internal/testutil/testutil.go
index e6602a6..2bba58e 100644
--- a/internal/testutil/testutil.go
+++ b/internal/testutil/testutil.go
@@ -57,6 +57,9 @@ func (sn Snake) Check(data []byte) (ok bool, n int) {
 			if !found { return false, n }
 		}
 	}
+	if n < len(data) {
+		return false, n
+	}
 	return true, n
 }
 
diff --git a/internal/testutil/testutil_test.go b/internal/testutil/testutil_test.go
index e8fa5e0..663831a 100644
--- a/internal/testutil/testutil_test.go
+++ b/internal/testutil/testutil_test.go
@@ -32,6 +32,8 @@ func TestSnakeA(test *testing.T) {
 	if ok { test.Fatal("false positive:", n) }
 	ok, n = snake.Check([]byte { 1, 6, 7, 3, 1, 4, 2, 5, 9 })
 	if ok { test.Fatal("false positive:", n) }
+	ok, n = snake.Check([]byte { 1, 6, 1, 2, 3, 4, 5, 9, 10})
+	if ok { test.Fatal("false positive:", n) }
 }
 
 func TestSnakeB(test *testing.T) {