diff --git a/gridworld/env.py b/gridworld/env.py
index 0858a87..d4dce45 100644
--- a/gridworld/env.py
+++ b/gridworld/env.py
@@ -229,7 +229,13 @@ def reset(self):
                 # blocks to remove have negative ids.
                 '', target_grid=self._task.target_grid - self._synthetic_init_grid
             )
-            self._synthetic_task.reset()
+        else:
+            self._synthetic_task = Task(
+                # create a synthetic task with only diff blocks. 
+                # blocks to remove have negative ids.
+                '', target_grid=self._task.target_grid
+            )
+        self._synthetic_task.reset()
 
         for block in set(self.world.placed):
             self.world.remove_block(block)
@@ -286,8 +292,11 @@ def step(self, action):
         obs['dialog'] = self._task.chat
         if self.vector_state:
             obs['grid'] = self.grid.copy().astype(np.int32)
-            obs['agentPos'] = np.array([x, y, z, pitch, yaw], dtype=np.float32)    
-        synthetic_grid = self.grid - self._synthetic_init_grid
+            obs['agentPos'] = np.array([x, y, z, pitch, yaw], dtype=np.float32)
+        if self._synthetic_init_grid is not None:
+            synthetic_grid = self.grid - self._synthetic_init_grid
+        else:
+            synthetic_grid = self.grid
         right_placement, wrong_placement, done = self._synthetic_task.step_intersection(synthetic_grid)
         done = done or (self.step_no == self.max_steps)
         if right_placement == 0: