diff --git a/tests/fba.py b/tests/fba.py
index 7c3561ae..94dc5b6f 100644
--- a/tests/fba.py
+++ b/tests/fba.py
@@ -10,7 +10,7 @@
 from dingo import MetabolicNetwork
 
 class TestFba(unittest.TestCase):
-    
+
     def test_fba_json(self):
 
         input_file_json = os.getcwd() + "/ext_data/e_coli_core.json"
@@ -21,7 +21,7 @@ def test_fba_json(self):
         self.assertTrue(abs(res[1] - 0.8739215067486387) < 1e-03)
 
     def test_fba_mat(self):
-        
+
         input_file_mat = os.getcwd() + "/ext_data/e_coli_core.mat"
         model = MetabolicNetwork.from_mat(input_file_mat)
         model.set_slow_mode()
@@ -49,26 +49,33 @@ def test_modify_medium(self):
         initial_medium = model.medium
         initial_fba = model.fba()[-1]
 
-        initial_medium_indices = {}
-        for reac in initial_medium:
-            initial_medium_indices[reac] = model.reactions.index(reac)
+        e_coli_core_medium_compound_indices = {
+            "EX_co2_e" : 46,
+            "EX_glc__D_e" : 51,
+            "EX_h_e" : 54,
+            "EX_h2o_e" : 55,
+            "EX_nh4_e" : 58,
+            "EX_o2_e" : 59,
+            "EX_pi_e" : 60
+        }
 
         glc_index = model.reactions.index("EX_glc__D_e")
+        o2_index = model.reactions.index("EX_o2_e")
 
         new_media = initial_medium.copy()
         new_media["EX_glc__D_e"] = 1.5
+        new_media["EX_o2_e"] = -0.5
 
         model.medium = new_media
 
         updated_media = model.medium
         updated_medium_indices = {}
         for reac in updated_media:
-            updated_medium_indices[reac] = model.reactions.index(reac)        
+            updated_medium_indices[reac] = model.reactions.index(reac)
 
-        if len(updated_media) == len(new_media):
-            self.assertTrue(updated_medium_indices == initial_medium_indices)
+        self.assertTrue(updated_medium_indices == e_coli_core_medium_compound_indices)
 
-        self.assertTrue(model.lb[glc_index] == -1.5)
+        self.assertTrue(model.lb[glc_index] == -1.5 and model.lb[o2_index] == 0.5)
 
         self.assertTrue(initial_fba - model.fba()[-1] > 0)