diff --git a/src/openmmio.py b/src/openmmio.py
index 038f2efa3..66be55a7e 100644
--- a/src/openmmio.py
+++ b/src/openmmio.py
@@ -1288,12 +1288,15 @@ def normal_modes(self, shot=0, optimize=True, for_hessian_target=False):
 
     def _update_positions(self, X1, disable_vsite):
         """A convenience method for updating the positions of the simulation context."""
+        # check if we have units
+        if isinstance(X1, numpy.ndarray):
+            X1 = X1 * angstrom
 
         if disable_vsite:
-            self.simulation.context.setPositions(X1 * angstrom)
+            self.simulation.context.setPositions(X1)
         else:
             # Create virtual sites before setting positions
-            mod = Modeller(self.pdb.topology, X1*angstrom)
+            mod = Modeller(self.pdb.topology, X1)
             mod.addExtraParticles(self.forcefield)
             self.simulation.context.setPositions(ResetVirtualSites_fast(mod.getPositions(), self.vsinfo))
 
diff --git a/src/smirnoffio.py b/src/smirnoffio.py
index dab32d84e..19376900b 100644
--- a/src/smirnoffio.py
+++ b/src/smirnoffio.py
@@ -534,7 +534,7 @@ def _update_positions(self, X1, disable_vsite):
         else:
             X1 = (X1 + [Vec3(0.0, 0.0, 0.0)] * n_v_sites) * angstrom
 
-        self.simulation.context.setPositions(X1 * angstrom)
+        self.simulation.context.setPositions(X1)
         self.simulation.context.computeVirtualSites()
 
     def interaction_energy(self, fraga, fragb):