diff --git a/pyerrors/obs.py b/pyerrors/obs.py
index 4247de8e..c24da74a 100644
--- a/pyerrors/obs.py
+++ b/pyerrors/obs.py
@@ -120,23 +120,24 @@ class Obs:
                     self.idl[name] = range(1, len(sample) + 1)
 
             self._value = 0
+            self.N = 0
             if means is not None:
                 for name, sample, mean in sorted(zip(names, samples, means)):
                     self.shape[name] = len(self.idl[name])
+                    self.N += self.shape[name]
                     if len(sample) != self.shape[name]:
                         raise Exception('Incompatible samples and idx for %s: %d vs. %d' % (name, len(sample), self.shape[name]))
                     self.r_values[name] = mean
                     self.deltas[name] = sample
-                self.N = sum(list(self.shape.values()))
             else:
                 for name, sample in sorted(zip(names, samples)):
                     self.shape[name] = len(self.idl[name])
+                    self.N += self.shape[name]
                     if len(sample) != self.shape[name]:
                         raise Exception('Incompatible samples and idx for %s: %d vs. %d' % (name, len(sample), self.shape[name]))
                     self.r_values[name] = np.mean(sample)
                     self.deltas[name] = sample - self.r_values[name]
                     self._value += self.shape[name] * self.r_values[name]
-                self.N = sum(list(self.shape.values()))
                 self._value /= self.N
 
             self.is_merged = {}