diff --git a/trackpy/motion.py b/trackpy/motion.py
index 8b4ee8f5..7432cad8 100644
--- a/trackpy/motion.py
+++ b/trackpy/motion.py
@@ -239,7 +239,7 @@ def emsd(traj, mpp, fps, max_lagtime=100, detail=False, pos_columns=None):
     if not detail:
         return results.set_index('lagt')['msd']
     # correctly compute the effective number of independent measurements
-    results['N'] = msds['N'].sum(level=1)
+    results['N'] = msds['N'].groupby(level=1).sum()
     return results