diff --git a/trackers/simple_long_tracking.py b/trackers/simple_long_tracking.py
index fa99bd58..58bcb76f 100644
--- a/trackers/simple_long_tracking.py
+++ b/trackers/simple_long_tracking.py
@@ -701,7 +701,7 @@ def __init__(self, alpha_array, circumference, Qs, D_x=0, D_y=0,*args, **kwargs)
         '''
         super(LinearMap, self).__init__(alpha_array, circumference,
                                         *args, **kwargs)
-        assert (np.isscalar(Qs)), "Qs has to be a scalar"
+        assert (len(np.atleast_1d(Qs)) == 1), "Qs can only have one entry!"
         self.Qs = Qs
         self.D_x = D_x
         self.D_y = D_y