diff --git a/pyerrors/obs.py b/pyerrors/obs.py
index 2dcab477..239efc5c 100644
--- a/pyerrors/obs.py
+++ b/pyerrors/obs.py
@@ -323,8 +323,8 @@ class Obs:
                     # Standard automatic windowing procedure
                     tau = self.S[e_name] / np.log((2 * self.e_n_tauint[e_name][gapsize::gapsize] + 1) / (2 * self.e_n_tauint[e_name][gapsize::gapsize] - 1))
                     g_w = np.exp(- np.arange(1, len(tau) + 1) / tau) - tau / np.sqrt(np.arange(1, len(tau) + 1) * e_N)
-                    for n in range(1, w_max):
-                        if g_w[n - 1] < 0 or n >= w_max - 1:
+                    for n in range(1, w_max // gapsize):
+                        if g_w[n - 1] < 0 or n >= w_max // gapsize - 1:
                             _compute_drho(gapsize * n)
                             n *= gapsize
                             self.e_tauint[e_name] = self.e_n_tauint[e_name][n] * (1 + (2 * n / gapsize + 1) / e_N) / (1 + 1 / e_N)  # Bias correction hep-lat/0306017 eq. (49)
diff --git a/tests/obs_test.py b/tests/obs_test.py
index 3f65ecee..824feb77 100644
--- a/tests/obs_test.py
+++ b/tests/obs_test.py
@@ -1152,3 +1152,7 @@ def test_nan_obs():
     o = pe.pseudo_Obs(1, .1, 'test')
     no = np.nan * o
     no.gamma_method()
+
+    o.idl['test'] = [1, 5] + list(range(7, 2002, 2))
+    no = np.NaN * o
+    no.gamma_method()