diff --git a/python/desc/imsim/imSim.py b/python/desc/imsim/imSim.py
index 8cf72a18..f84b3d94 100644
--- a/python/desc/imsim/imSim.py
+++ b/python/desc/imsim/imSim.py
@@ -183,7 +183,7 @@ def sources_from_list(object_lines, obs_md, phot_params, file_name):
     rotation_angle = np.zeros(num_objects, dtype=float)
     fits_image_file = dict()
 
-    unique_id = np.zeros(num_objects, dtype=int)
+    unique_id = [None]*num_objects
     object_type = np.zeros(num_objects, dtype=int)
 
     i_obj = -1
@@ -192,7 +192,7 @@ def sources_from_list(object_lines, obs_md, phot_params, file_name):
         if params[0] != 'object':
             continue
         i_obj += 1
-        unique_id[i_obj] = int(params[1])
+        unique_id[i_obj] = params[1]
         ra_phosim[i_obj] = float(params[2])
         dec_phosim[i_obj] = float(params[3])
         mag_norm[i_obj] = float(params[4])
diff --git a/tests/test_instcat_parser.py b/tests/test_instcat_parser.py
index df6b797d..d00873d4 100644
--- a/tests/test_instcat_parser.py
+++ b/tests/test_instcat_parser.py
@@ -151,11 +151,11 @@ def test_object_extraction_stars(self):
                                                         phot_params,
                                                         self.phosim_file)
 
-        id_arr = np.zeros(len(gs_object_arr), dtype=int)
+        id_arr = [None]*len(gs_object_arr)
         for i_obj in range(len(gs_object_arr)):
             id_arr[i_obj] = gs_object_arr[i_obj].uniqueId
 
-        truth_dtype = np.dtype([('uniqueId', int), ('x_pupil', float), ('y_pupil', float),
+        truth_dtype = np.dtype([('uniqueId', str, 200), ('x_pupil', float), ('y_pupil', float),
                                 ('sedFilename', str, 200), ('magNorm', float),
                                 ('raJ2000', float), ('decJ2000', float),
                                 ('pmRA', float), ('pmDec', float),
@@ -253,11 +253,11 @@ def test_object_extraction_galaxies(self):
                                                         phot_params,
                                                         galaxy_phosim_file)
 
-        id_arr = np.zeros(len(gs_object_arr), dtype=int)
+        id_arr = [None]*len(gs_object_arr)
         for i_obj in range(len(gs_object_arr)):
             id_arr[i_obj] = gs_object_arr[i_obj].uniqueId
 
-        truth_dtype = np.dtype([('uniqueId', int), ('x_pupil', float), ('y_pupil', float),
+        truth_dtype = np.dtype([('uniqueId', str, 200), ('x_pupil', float), ('y_pupil', float),
                                 ('sedFilename', str, 200), ('magNorm', float),
                                 ('raJ2000', float), ('decJ2000', float),
                                 ('redshift', float), ('gamma1', float),
@@ -424,9 +424,10 @@ def test_validate_phosim_object_list(self):
         message = wa[desired_warning_dex].message.args[0]
 
         # these are the objects that should be omitted
-        bad_unique_ids = set([34307989098524, 811883374597,
+        bad_unique_ids = set([str(x) for x in
+                              [34307989098524, 811883374597,
                               811883374596, 956090392580,
-                              34307989098523, 34304522113056])
+                               34307989098523, 34304522113056]])
 
         self.assertIn('Omitted 6 suspicious objects', message)
         self.assertIn('4 had galactic_Av', message)
diff --git a/tests/tiny_instcat.txt b/tests/tiny_instcat.txt
index fc9c6553..fd7be54f 100644
--- a/tests/tiny_instcat.txt
+++ b/tests/tiny_instcat.txt
@@ -18,7 +18,7 @@ seed 161899
 seeing 0.7613760
 sunalt -59.1098785
 vistime 33.0000000
-object 1046817878020 31.2400746 -10.09365 29.3370237 starSED/phoSimMLT/lte033-4.5-1.0a+0.4.BT-Settl.spec.gz 0 0 0 0 0 0 point none CCM 0.0635117705 3.1
+object MS_567_8 31.2400746 -10.09365 29.3370237 starSED/phoSimMLT/lte033-4.5-1.0a+0.4.BT-Settl.spec.gz 0 0 0 0 0 0 point none CCM 0.0635117705 3.1
 object 811883456516 31.2338048 -10.0578789 26.9267056 starSED/phoSimMLT/lte027-2.0-0.0a+0.0.BT-Settl.spec.gz 0 0 0 0 0 0 point none CCM 0.0630700848 3.1
 object 1046816368644 31.2371416 -10.0554117 28.7153047 starSED/phoSimMLT/lte027-2.0-0.0a+0.0.BT-Settl.spec.gz 0 0 0 0 0 0 point none CCM 0.0630379733 3.1
 object 1046816356356 31.2305508 -10.0620637 26.7932237 starSED/phoSimMLT/lte033-4.5-1.0a+0.4.BT-Settl.spec.gz 0 0 0 0 0 0 point none CCM 0.0631214889 3.1