diff --git a/.circleci/config.yml b/.circleci/config.yml
index 064a5e0..c5503e0 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -148,6 +148,8 @@ jobs:
                 command: |
                       source venv_test/bin/activate
                       pip install "grid2op~=1.7.0"
+                      sed -i -e 's/np.int)/np.int32)/g' venv_test/lib/python3.*/site-packages/grid2op/Backend/PandaPowerBackend.py
+                      sed -i -e 's/np.bool/np.bool_/g' venv_test/lib/python3.*/site-packages/grid2op/dtypes.py
                       python -m unittest lightsim2grid/tests/test_compat_legacy_grid2op.py
             - run:
                 name: legacy test (grid2op ~1.8)
@@ -159,6 +161,8 @@ jobs:
                 name: legacy test (grid2op ~1.9)
                 command: |
                       source venv_test/bin/activate
+                      pip uninstall gym
+                      pip install gymnasium
                       pip install "grid2op~=1.9.0"
                       python -m unittest lightsim2grid/tests/test_compat_legacy_grid2op.py
             - run:
@@ -171,6 +175,8 @@ jobs:
                 name: legacy test (grid2op 0.9.1.post1 - l2rpn wcci 2020)
                 command: |
                       source venv_test/bin/activate
+                      pip uninstall gymnasium
+                      pip install gym
                       pip install grid2op==0.9.1.post1
                       sed -i -e 's/np.int)/np.int32)/g' venv_test/lib/python3.*/site-packages/grid2op/Backend/PandaPowerBackend.py
                       sed -i -e 's/np.bool/np.bool_/g' venv_test/lib/python3.*/site-packages/grid2op/dtypes.py
@@ -199,7 +205,9 @@ jobs:
                 name: legacy test (grid2op 1.9.0 - l2rpn Delft 2023)
                 command: |
                       source venv_test/bin/activate
-                      ppip install grid2op==1.9.0
+                      pip uninstall gym
+                      pip install gymnasium
+                      pip install grid2op==1.9.0
                       python -m unittest lightsim2grid/tests/test_compat_legacy_grid2op.py
             - run:
                 name: legacy test (grid2op 1.9.1 - l2rpn IDF 2023)
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 7250342..45809a2 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -83,6 +83,8 @@ gridmodel.get_Bf()                  gridmodel.get_Bf_solver()
 - [FIXED] basic backward compatibility is ensured and tested for grid2op >= 0.9.1.post1
   Not all features are tested and only 1.x versions are tested 
   (ie 1.1 or 1.2 but not 1.2.1, 1.2.2, 1.2.3 etc.)
+- [FIXED] a bug when using `LightSimBackend` with some old (but not too old) grid2op
+  versions.
 - [ADDED] it is now possible to deactivate the support for shunts by 
   subclassing the LightSimBackend class and setting the `shunts_data_available`
   to `False`
diff --git a/lightsim2grid/tests/test_compat_legacy_grid2op.py b/lightsim2grid/tests/test_compat_legacy_grid2op.py
index 0cd3912..98d7ad5 100644
--- a/lightsim2grid/tests/test_compat_legacy_grid2op.py
+++ b/lightsim2grid/tests/test_compat_legacy_grid2op.py
@@ -27,11 +27,26 @@
 except ImportError as exc_:
     print(exc_)
     GYM_AVAIL = False
+
+try:
+    import gymnasium
+    GYMANSIUM_AVAIL = True
+except ImportError:
+    GYMANSIUM_AVAIL = False
     
+try: 
+    import gym
+    GYM_AVAIL = True
+except ImportError:
+    GYM_AVAIL = False
+    
+
 from lightsim2grid import LightSimBackend
 
 
 GRID2OP_VER_FIXED_REWARD = version.parse("1.5.0")    
+GRID2OP_VER_NEW_GYM_API = version.parse("1.8.0")    
+            
             
 class TestEnvironmentBasic(unittest.TestCase):          
     def setUp(self) -> None:
@@ -163,8 +178,14 @@ def test_runner(self):
 class TestBasicEnvironmentGym(unittest.TestCase):
     def setUp(self) -> None:
         if not GYM_AVAIL:
-            self.skipTest("gym is not installed (gymnasium did not exist in 1.6.4)")
+            self.skipTest("gym is not installed (no gym compat before 1.3.0)")
         TestEnvironmentBasic.setUp(self)
+        self.new_gym_api = False
+        if GYMANSIUM_AVAIL:
+            self.new_gym_api = True
+        if version.parse(grid2op.__version__) >= GRID2OP_VER_NEW_GYM_API:
+            self.new_gym_api = True
+        
 
     def tearDown(self) -> None:
         self.env.close()
@@ -172,7 +193,10 @@ def tearDown(self) -> None:
     
     def _aux_run_envs(self, act, env_gym):
         for i in range(10):
-            obs_in, reward, done, info = env_gym.step(act)
+            if self.new_gym_api:
+                obs_in, reward, done, truncated, info = env_gym.step(act)
+            else:
+                obs_in, reward, done, info = env_gym.step(act)
             if i < 3:
                 assert obs_in["timestep_overflow"][self.line_id] == i + 1, f"error for step {i}: {obs_in['timestep_overflow'][self.line_id]}"
             else: