diff --git a/predicators/explorers/active_sampler_explorer.py b/predicators/explorers/active_sampler_explorer.py
index 6f23e7ebff..898f772a35 100644
--- a/predicators/explorers/active_sampler_explorer.py
+++ b/predicators/explorers/active_sampler_explorer.py
@@ -57,7 +57,10 @@ def __init__(self, predicates: Set[Predicate],
         self._last_executed_nsrt: Optional[_GroundNSRT] = None
         self._nsrt_to_explorer_sampler = nsrt_to_explorer_sampler
         self._seen_train_task_idxs = seen_train_task_idxs
-        self._task_plan_cache: Dict[_TaskID, List[_GroundSTRIPSOperator]] = {}
+        # If the plan is None, that means none can be found, e.g., due to
+        # timeouts or dead-ends.
+        self._task_plan_cache: Dict[
+            _TaskID, Optional[List[_GroundSTRIPSOperator]]] = {}
         self._task_plan_calls_since_replan: Dict[_TaskID, int] = {}
         self._sorted_options = sorted(options, key=lambda o: o.name)
 
@@ -410,17 +413,20 @@ def _score_ground_op_planning_progress(
         replan_task_ids = [("replan", i) for i in range(len(num_replan_tasks))]
         for task_id in train_task_ids + replan_task_ids:
             plan = self._get_task_plan_for_task(task_id, ground_op_costs)
-            task_plan_costs = []
-            for op in plan:
-                op_cost = ground_op_costs.get(op, self._default_cost)
-                task_plan_costs.append(op_cost)
-            plan_costs.append(sum(task_plan_costs))
+            # If no plan can be found for a task, the task is just ignored.
+            if plan is not None:
+                task_plan_costs = []
+                for op in plan:
+                    op_cost = ground_op_costs.get(op, self._default_cost)
+                    task_plan_costs.append(op_cost)
+                plan_costs.append(sum(task_plan_costs))
         return -sum(plan_costs)  # higher scores are better
 
     def _get_task_plan_for_task(
         self, task_id: _TaskID, ground_op_costs: Dict[_GroundSTRIPSOperator,
                                                       float]
-    ) -> List[_GroundSTRIPSOperator]:
+    ) -> Optional[List[_GroundSTRIPSOperator]]:
+        """Returns None if no task plan can be found."""
         # Optimization: only re-plan at a certain frequency.
         replan_freq = CFG.active_sampler_explorer_replan_frequency
         if task_id not in self._task_plan_calls_since_replan or \
@@ -433,18 +439,22 @@ def _get_task_plan_for_task(
                 "train": self._train_tasks,
                 "replan": self._replanning_tasks,
             }[task_type][task_idx]
-            plan, _, _ = run_task_plan_once(
-                task,
-                self._nsrts,
-                self._predicates,
-                self._types,
-                timeout,
-                self._seed,
-                task_planning_heuristic=task_planning_heuristic,
-                ground_op_costs=ground_op_costs,
-                default_cost=self._default_cost,
-                max_horizon=np.inf)
-            self._task_plan_cache[task_id] = [n.op for n in plan]
+            try:
+                plan, _, _ = run_task_plan_once(
+                    task,
+                    self._nsrts,
+                    self._predicates,
+                    self._types,
+                    timeout,
+                    self._seed,
+                    task_planning_heuristic=task_planning_heuristic,
+                    ground_op_costs=ground_op_costs,
+                    default_cost=self._default_cost,
+                    max_horizon=np.inf)
+                self._task_plan_cache[task_id] = [n.op for n in plan]
+            except (PlanningFailure, PlanningTimeout):  # pragma: no cover
+                logging.info("WARNING: task planning failed in the explorer.")
+                self._task_plan_cache[task_id] = None
 
         self._task_plan_calls_since_replan[task_id] += 1
         return self._task_plan_cache[task_id]