diff --git a/yaff/sampling/harmonic.py b/yaff/sampling/harmonic.py
index 0286f684..7d673fab 100644
--- a/yaff/sampling/harmonic.py
+++ b/yaff/sampling/harmonic.py
@@ -166,7 +166,7 @@ def estimate_elastic(ff, eps=1e-4, do_frozen=False, ridge=1e-4):
     """
     cell = ff.system.cell
     if cell.nvec == 0:
-        raise VelueError('The elastic constants can only be computed if the system is periodic.')
+        raise ValueError('The elastic constants can only be computed if the system is periodic.')
     dof = StrainCellDOF(ff, do_frozen=do_frozen)
     vol0 = cell.volume
     if do_frozen:
diff --git a/yaff/sampling/npt.py b/yaff/sampling/npt.py
index fee1910a..bc12022e 100644
--- a/yaff/sampling/npt.py
+++ b/yaff/sampling/npt.py
@@ -635,7 +635,10 @@ def post(self, iterative, chainvel0 = None):
         # propagate the barostat
         self.baro(iterative, chainvel0)
         # calculate the correction due to the barostat alone
-        self.econs_correction = self.press*iterative.ff.system.cell.volume + self._compute_ekin_baro()
+        self.econs_correction = self._compute_ekin_baro()
+        # add the PV term if the volume is not constrained
+        if not self.vol_constraint:
+            self.econs_correction += self.press*iterative.ff.system.cell.volume
         if self.baro_thermo is not None:
             # add the correction due to the barostat thermostat
             self.econs_correction += self.baro_thermo.chain.get_econs_correction()
diff --git a/yaff/sampling/verlet.py b/yaff/sampling/verlet.py
index 65b00b64..9d08f0ff 100644
--- a/yaff/sampling/verlet.py
+++ b/yaff/sampling/verlet.py
@@ -493,7 +493,7 @@ def get(self):
         if self.counter > 0:
             ekin_var = self.ekin_sumsq/self.counter - (self.ekin_sum/self.counter)**2
             max_contrib = max(self.ekin_sumsq/self.counter, (self.ekin_sum/self.counter)**2)
-            if np.abs(ekin_var)/max_contrib > 10.*self.eps:
+            if max_contrib > 10.*self.eps and np.abs(ekin_var)/max_contrib > 10.*self.eps:
             # only calculate further if the variance is substantially different from zero,
             # to mediate precision errors
                 econs_var = self.econs_sumsq/self.counter - (self.econs_sum/self.counter)**2