diff --git a/tests/test_field_distortion.py b/tests/test_field_distortion.py
index 213d913a7c384bd4c87e2b944ee8171b47b8c9dc..0f77edbe4062a24d36f90bb07963045b700b4bc9 100644
--- a/tests/test_field_distortion.py
+++ b/tests/test_field_distortion.py
@@ -238,7 +238,7 @@ def field_distortion_model(
             # if iccd!=1: continue
             iccdKey = "ccd" + str("0%d" % (iccd))[-2:]
             # open the ditortion data
-            ipsfDatn = cat_dir + "ccd%d_%s.dat" % (iccd, iwaveKey)
+            ipsfDatn = os.path.join(cat_dir, "ccd%d_%s.dat" % (iccd, iwaveKey))
             ipsfDat = Table.read(ipsfDatn, format="ascii")
 
             ixwList, iywList = [], []