diff --git a/pyerrors/obs.py b/pyerrors/obs.py
index 7d8f9911..d1e4f5c7 100644
--- a/pyerrors/obs.py
+++ b/pyerrors/obs.py
@@ -278,7 +278,7 @@ class Obs:
 
             def _compute_drho(i):
                 tmp = (self.e_rho[e_name][i + 1:w_max]
-                       + np.concatenate([self.e_rho[e_name][i - 1:None if i - w_max // 2 <= 0 else (2 * i - (2 * w_max) // 2):-1],
+                       + np.concatenate([self.e_rho[e_name][i - 1:None if i - (w_max - 1) // 2 <= 0 else (2 * i - (2 * w_max) // 2):-1],
                                          self.e_rho[e_name][1:max(1, w_max - 2 * i)]])
                        - 2 * self.e_rho[e_name][i] * self.e_rho[e_name][1:w_max - i])
                 self.e_drho[e_name][i] = np.sqrt(np.sum(tmp ** 2) / e_N)
diff --git a/tests/data/compute_drho_fails.json.gz b/tests/data/compute_drho_fails.json.gz
new file mode 100644
index 00000000..3fc335bd
Binary files /dev/null and b/tests/data/compute_drho_fails.json.gz differ
diff --git a/tests/obs_test.py b/tests/obs_test.py
index a77c6863..c8b741c2 100644
--- a/tests/obs_test.py
+++ b/tests/obs_test.py
@@ -1282,3 +1282,8 @@ def test_f_string_obs():
     print(f"{o1:+3}")
     print(f"{o1:-1}")
     print(f"{o1: 8}")
+
+def test_compute_drho_fails():
+    obs = pe.input.json.load_json("tests/data/compute_drho_fails.json.gz")
+    obs.gm()
+    assert np.isclose(obs.dvalue, 0.0022150779611891094)