diff --git a/lisainstrument/instrument.py b/lisainstrument/instrument.py
index 762149b4f5c8430b2190397ef71f8f999ae5fb2e..1062db019c1f58802a8099bd4abb1acd737119f0 100644
--- a/lisainstrument/instrument.py
+++ b/lisainstrument/instrument.py
@@ -1547,9 +1547,11 @@ class Instrument:
         logger.debug("Solving iteratively (tolerance=%s s, maxiter=%s)",
             self.clockinv_tolerance, self.clockinv_maxiter)
 
-        niter = 0
+        # Drop samples at the edges to compute error
+        edge = min(100, len(timer_deviations) // 2 - 1)
         error = 0
-        edge = 100 # drop samples at the edges to check convergence
+
+        niter = 0
         next_inverse = timer_deviations
         while not niter or error > self.clockinv_tolerance:
             if niter >= self.clockinv_maxiter:
diff --git a/tests/test_instrument.py b/tests/test_instrument.py
index 7b2d08b415a974207548ff055bb28540f88bb151..54adb731c58fedd3dcdf3855f0bd5df966c53c42 100644
--- a/tests/test_instrument.py
+++ b/tests/test_instrument.py
@@ -16,7 +16,6 @@ def test_run_no_aafilter():
     instru = Instrument(size=100, aafilter=None)
     instru.simulate()
 
-@pytest.mark.skip(reason="do not know why this fails")
 def test_run_no_upsampling():
     """Test that simulations can run with no filter."""
     instru = Instrument(size=100, physics_upsampling=1, aafilter=None)