diff --git a/orbitize/hipparcos.py b/orbitize/hipparcos.py
index fda8fde5..a3cab7d9 100644
--- a/orbitize/hipparcos.py
+++ b/orbitize/hipparcos.py
@@ -286,14 +286,14 @@ 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]
         self.R = iad[5]  # abscissa residual [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]