diff --git a/rllib/agents/mbmpo/mbmpo_torch_policy.py b/rllib/agents/mbmpo/mbmpo_torch_policy.py
index 06e65042e..5dc03435c 100644
--- a/rllib/agents/mbmpo/mbmpo_torch_policy.py
+++ b/rllib/agents/mbmpo/mbmpo_torch_policy.py
@@ -1,4 +1,5 @@
 import gym
+from gym.spaces import Box, Discrete
 import logging
 from typing import Tuple, Type
 
@@ -13,6 +14,7 @@ from ray.rllib.models.modelv2 import ModelV2
 from ray.rllib.models.torch.torch_action_dist import TorchDistributionWrapper
 from ray.rllib.policy.policy import Policy
 from ray.rllib.policy.policy_template import build_policy_class
+from ray.rllib.utils.error import UnsupportedSpaceException
 from ray.rllib.utils.framework import try_import_torch
 from ray.rllib.utils.torch_ops import apply_grad_clipping
 from ray.rllib.utils.typing import TrainerConfigDict
@@ -22,6 +24,35 @@ torch, nn = try_import_torch()
 logger = logging.getLogger(__name__)
 
 
+def validate_spaces(policy: Policy, observation_space: gym.spaces.Space,
+                    action_space: gym.spaces.Space,
+                    config: TrainerConfigDict) -> None:
+    """Validates the observation- and action spaces used for the Policy.
+
+    Args:
+        policy (Policy): The policy, whose spaces are being validated.
+        observation_space (gym.spaces.Space): The observation space to
+            validate.
+        action_space (gym.spaces.Space): The action space to validate.
+        config (TrainerConfigDict): The Policy's config dict.
+
+    Raises:
+        UnsupportedSpaceException: If one of the spaces is not supported.
+    """
+    # Only support single Box or single Discrete spaces.
+    if not isinstance(action_space, (Box, Discrete)):
+        raise UnsupportedSpaceException(
+            "Action space ({}) of {} is not supported for "
+            "MB-MPO. Must be [Box|Discrete].".format(action_space, policy))
+    # If Box, make sure it's a 1D vector space.
+    elif isinstance(action_space, Box) and len(action_space.shape) > 1:
+        raise UnsupportedSpaceException(
+            "Action space ({}) of {} has multiple dimensions "
+            "{}. ".format(action_space, policy, action_space.shape) +
+            "Consider reshaping this into a single dimension Box space "
+            "or using the multi-agent API.")
+
+
 def make_model_and_action_dist(
         policy: Policy,
         obs_space: gym.spaces.Space,
diff --git a/rllib/agents/mbmpo/model_ensemble.py b/rllib/agents/mbmpo/model_ensemble.py
index 2bb9513da..f7cb35b6f 100644
--- a/rllib/agents/mbmpo/model_ensemble.py
+++ b/rllib/agents/mbmpo/model_ensemble.py
@@ -136,6 +136,8 @@ class DynamicsEnsembleCustomModel(TorchModelV2, nn.Module):
                 obs_space.low[0],
                 obs_space.high[0],
                 shape=(obs_space.shape[0] + action_space.shape[0], ))
+        else:
+            raise NotImplementedError
         super(DynamicsEnsembleCustomModel, self).__init__(
             input_space, action_space, num_outputs, model_config, name)
 
diff --git a/rllib/agents/sac/sac_tf_policy.py b/rllib/agents/sac/sac_tf_policy.py
index 83fa076ed..e4cc080af 100644
--- a/rllib/agents/sac/sac_tf_policy.py
+++ b/rllib/agents/sac/sac_tf_policy.py
@@ -652,7 +652,7 @@ def validate_spaces(policy: Policy, observation_space: gym.spaces.Space,
     Raises:
         UnsupportedSpaceException: If one of the spaces is not supported.
     """
-    # Only support single Box or single Discreete spaces.
+    # Only support single Box or single Discrete spaces.
     if not isinstance(action_space, (Box, Discrete, Simplex)):
         raise UnsupportedSpaceException(
             "Action space ({}) of {} is not supported for "