diff --git a/pytorch_lightning/trainer/training_loop.py b/pytorch_lightning/trainer/training_loop.py
index 7a12d7e766dae..95ba6d5306309 100644
--- a/pytorch_lightning/trainer/training_loop.py
+++ b/pytorch_lightning/trainer/training_loop.py
@@ -16,7 +16,7 @@
 from contextlib import contextmanager, suppress
 from copy import copy
 from functools import partial, update_wrapper
-from typing import Any, Callable, Dict, List, Optional, Tuple, Union
+from typing import Any, Callable, Dict, List, Mapping, Optional, Tuple, Union
 
 import numpy as np
 import torch
@@ -269,6 +269,16 @@ def _check_training_step_output(self, training_step_output):
             if training_step_output.grad_fn is None:
                 # TODO: Find why - RuntimeError: Expected to mark a variable ready only once ...
                 raise MisconfigurationException("In manual optimization, `training_step` should not return a Tensor")
+        elif self.trainer.lightning_module.automatic_optimization:
+            if not any((
+                isinstance(training_step_output, torch.Tensor),
+                (isinstance(training_step_output, Mapping)
+                 and 'loss' in training_step_output), training_step_output is None
+            )):
+                raise MisconfigurationException(
+                    "In automatic optimization, `training_step` must either return a Tensor, "
+                    "a dict with key 'loss' or None (where the step will be skipped)."
+                )
 
     def training_step(self, split_batch, batch_idx, opt_idx, hiddens):
         # give the PL module a result for logging
diff --git a/tests/trainer/loops/test_training_loop.py b/tests/trainer/loops/test_training_loop.py
index da4ecbe5a9f05..a2706e5d37bc0 100644
--- a/tests/trainer/loops/test_training_loop.py
+++ b/tests/trainer/loops/test_training_loop.py
@@ -11,10 +11,13 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import re
+
 import pytest
 import torch
 
 from pytorch_lightning import seed_everything, Trainer
+from pytorch_lightning.utilities.exceptions import MisconfigurationException
 from tests.helpers import BoringModel
 
 
@@ -142,3 +145,24 @@ def validation_step(self, *args):
     assert trainer.current_epoch == 0
     assert trainer.global_step == 5
     assert model.validation_called_at == (0, 4)
+
+
+@pytest.mark.parametrize(['output'], [(5., ), ({'a': 5}, )])
+def test_warning_invalid_trainstep_output(tmpdir, output):
+
+    class TestModel(BoringModel):
+
+        def training_step(self, batch, batch_idx):
+            return output
+
+    model = TestModel()
+
+    trainer = Trainer(default_root_dir=tmpdir, fast_dev_run=1)
+    with pytest.raises(
+        MisconfigurationException,
+        match=re.escape(
+            "In automatic optimization, `training_step` must either return a Tensor, "
+            "a dict with key 'loss' or None (where the step will be skipped)."
+        )
+    ):
+        trainer.fit(model)