diff --git a/xgm.py b/xgm.py
index 5ef58ba53f39a93cc68c2612510cb6f4c7d188f3..91f6f2fd55a0dabb206f7a182d620b230e0b24e7 100644
--- a/xgm.py
+++ b/xgm.py
@@ -478,6 +478,7 @@ def calibrateTIM(data, rollingWindow=200, mcp=1, plot=False, use_apd=True, intst
     start = 0
     stop = None
     npulses = data['npulses_sase3']
+    ntrains = npulses.shape[0]
     if not np.all(npulses == npulses[0]):
         start = np.argmax(npulses.values)
         stop = ntrains + np.argmax(npulses.values[::-1]) - 1