diff --git a/orbitize/hipparcos.py b/orbitize/hipparcos.py
index 64f61412..98e96bf7 100644
--- a/orbitize/hipparcos.py
+++ b/orbitize/hipparcos.py
@@ -291,8 +291,6 @@ def __init__(
         else:
             iad = np.transpose(np.loadtxt(path_to_iad_file))
 
-        n_lines = len(iad)
-
         times = iad[1] + 1991.25
         self.cos_phi = iad[3]  # scan direction
         self.sin_phi = iad[4]
@@ -301,6 +299,8 @@ def __init__(
             self.R += 0.140  # [mas]
         self.eps = iad[6]  # error on abscissa residual [mas]
 
+        n_lines = len(self.eps)
+
         # reject negative errors (scans that were rejected by Hipparcos team)
         good_scans = np.where(self.eps > 0)[0]