diff --git a/diagonals/zonmean.py b/diagonals/zonmean.py
index 51833351d69ead5fbd46008c3a773e287fc688bb..24dc42f633debc99120149a4246fddcec213e091 100644
--- a/diagonals/zonmean.py
+++ b/diagonals/zonmean.py
@@ -148,8 +148,8 @@ def _zonal_mean_cpu(variable, weight, latitude):
             if weight[i, j] == 0:
                 continue
             bin_value = int(round(latitude[i, j]) + 90)
-            weights[bin_value] += weight[i, j]
-            total[bin_value] += variable[i, j] * weight[i, j]
+            weights[bin_value-1] += weight[i, j]
+            total[bin_value-1] += variable[i, j] * weight[i, j]
     return total / weights