diff --git a/flatland/envs/env_utils.py b/flatland/envs/env_utils.py
index ee2c263711906370900c14a6030d57ada573c2ea..626ee5f0b4e1ab22349240bf15461f6df97cea7d 100644
--- a/flatland/envs/env_utils.py
+++ b/flatland/envs/env_utils.py
@@ -71,30 +71,42 @@ def validate_new_transition(rail_trans, rail_array, prev_pos, current_pos, new_p
     return rail_trans.is_valid(new_trans)
 
 
-def position_to_coordinate(width, position):
+def position_to_coordinate(depth, position):
     """
+         [ (0,0) (0,1) ..  (0,w)
+           (1,0) (1,1)     (1,w)
+           ...
+           (d,0) (d,1)     (d,w) ]
 
-    :param width:
+         -->
+
+         [ 0      1    ..   w
+           w+1    w+2  ..   2w
+           ...
+           d*w+1  d*w+
+
+    :param depth:
     :param position:
     :return:
     """
     coords = ()
     for p in position:
-        coords = coords + ((int(p) % width, int(p) // width),)  # changed x_dim to y_dim
+        coords = coords + ((int(p) % depth, int(p) // depth),)  # changed x_dim to y_dim
     return coords
 
 
-def coordinate_to_position(width, coords):
+def coordinate_to_position(depth, coords):
     """
+    Helper function to
 
-    :param width:
+    :param depth:
     :param coords:
     :return:
     """
     position = np.empty(len(coords), dtype=int)
     idx = 0
     for t in coords:
-        position[idx] = int(t[1] * width + t[0])
+        position[idx] = int(t[1] * depth + t[0])
         idx += 1
     return position
 
diff --git a/flatland/envs/predictions.py b/flatland/envs/predictions.py
index 3fda7378771bf2b5cd20c7d065c9454d3e5629a5..43909669a67c527ae6fb935e22810eb47c9608cd 100644
--- a/flatland/envs/predictions.py
+++ b/flatland/envs/predictions.py
@@ -112,7 +112,7 @@ class ShortestPathPredictorForRailEnv(PredictionBuilder):
         agents = self.env.agents
         if handle:
             agents = [self.env.agents[handle]]
-        assert custom_args
+        assert custom_args is not None
         distance_map = custom_args.get('distance_map')
         assert distance_map is not None
 
diff --git a/tests/test_env_utils.py b/tests/test_env_utils.py
new file mode 100644
index 0000000000000000000000000000000000000000..b6764f59ffd522a87bdeee990cace285a1d73cd6
--- /dev/null
+++ b/tests/test_env_utils.py
@@ -0,0 +1,21 @@
+import numpy as np
+
+from flatland.envs.env_utils import position_to_coordinate, coordinate_to_position
+
+depth_to_test = 5
+positions_to_test = [0, 5, 1, 6, 20, 30]
+coordinates_to_test = [[0, 0], [0, 1], [1, 0], [1, 1], [0, 4], [0, 6]]
+
+
+def test_position_to_coordinate():
+    actual_coordinates = position_to_coordinate(depth_to_test, positions_to_test)
+    expected_coordinates = coordinates_to_test
+    assert np.array_equal(actual_coordinates, expected_coordinates), \
+        "converted positions {}, expected {}".format(actual_coordinates, expected_coordinates)
+
+
+def test_coordinate_to_position():
+    actual_positions = coordinate_to_position(depth_to_test, coordinates_to_test)
+    expected_positions = positions_to_test
+    assert np.array_equal(actual_positions, expected_positions), \
+        "converted positions {}, expected {}".format(actual_positions, expected_positions)