diff --git a/pymc3/backends/ndarray.py b/pymc3/backends/ndarray.py
index 4f173acee7..7a523175df 100644
--- a/pymc3/backends/ndarray.py
+++ b/pymc3/backends/ndarray.py
@@ -6,12 +6,16 @@
 import json
 import os
 import shutil
+from typing import Optional, Dict, Any
 
 import numpy as np
-from ..backends import base
+from pymc3.backends import base
+from pymc3.backends.base import MultiTrace
+from pymc3.model import Model
+from pymc3.exceptions import TraceDirectoryError
 
 
-def save_trace(trace, directory=None, overwrite=False):
+def save_trace(trace: MultiTrace, directory: Optional[str]=None, overwrite=False) -> str:
     """Save multitrace to file.
 
     TODO: Also save warnings.
@@ -54,7 +58,7 @@ def save_trace(trace, directory=None, overwrite=False):
     return directory
 
 
-def load_trace(directory, model=None):
+def load_trace(directory: str, model=None) -> MultiTrace:
     """Loads a multitrace that has been written to file.
 
     A the model used for the trace must be passed in, or the command
@@ -72,17 +76,21 @@ def load_trace(directory, model=None):
     pm.Multitrace that was saved in the directory
     """
     straces = []
-    for directory in glob.glob(os.path.join(directory, '*')):
-        if os.path.isdir(directory):
-            straces.append(SerializeNDArray(directory).load(model))
+    for subdir in glob.glob(os.path.join(directory, '*')):
+        if os.path.isdir(subdir):
+            straces.append(SerializeNDArray(subdir).load(model))
+    if not straces:
+        raise TraceDirectoryError("%s is not a PyMC3 saved chain directory." % directory)
     return base.MultiTrace(straces)
 
 
 class SerializeNDArray:
     metadata_file = 'metadata.json'
     samples_file = 'samples.npz'
+    metadata_path = None # type: str
+    samples_path = None # type: str
 
-    def __init__(self, directory):
+    def __init__(self, directory: str):
         """Helper to save and load NDArray objects"""
         self.directory = directory
         self.metadata_path = os.path.join(self.directory, self.metadata_file)
@@ -126,8 +134,11 @@ def save(self, ndarray):
 
         np.savez_compressed(self.samples_path, **ndarray.samples)
 
-    def load(self, model):
+    def load(self, model: Model) -> 'NDArray':
         """Load the saved ndarray from file"""
+        if not os.path.exists(self.samples_path) or not os.path.exists(self.metadata_path):
+            raise TraceDirectoryError("%s is not a trace directory" % self.directory)
+
         new_trace = NDArray(model=model)
         with open(self.metadata_path, 'r') as buff:
             metadata = json.load(buff)
@@ -165,7 +176,7 @@ def __init__(self, name=None, model=None, vars=None, test_point=None):
 
     # Sampling methods
 
-    def setup(self, draws, chain, sampler_vars=None):
+    def setup(self, draws, chain, sampler_vars=None) -> None:
         """Perform chain-specific setup.
 
         Parameters
@@ -204,7 +215,7 @@ def setup(self, draws, chain, sampler_vars=None):
         if self._stats is None:
             self._stats = []
             for sampler in sampler_vars:
-                data = dict()
+                data = dict() # type: Dict[str, np.ndarray]
                 self._stats.append(data)
                 for varname, dtype in sampler.items():
                     data[varname] = np.zeros(draws, dtype=dtype)
@@ -218,7 +229,7 @@ def setup(self, draws, chain, sampler_vars=None):
                     new = np.zeros(draws, dtype=dtype)
                     data[varname] = np.concatenate([old, new])
 
-    def record(self, point, sampler_stats=None):
+    def record(self, point, sampler_stats=None) -> None:
         """Record results of a sampling iteration.
 
         Parameters
@@ -261,7 +272,7 @@ def __len__(self):
             return 0
         return self.draw_idx
 
-    def get_values(self, varname, burn=0, thin=1):
+    def get_values(self, varname: str, burn=0, thin=1) -> np.ndarray:
         """Get values from trace.
 
         Parameters
@@ -302,7 +313,7 @@ def _slice(self, idx):
 
         return sliced
 
-    def point(self, idx):
+    def point(self, idx) -> Dict[str, Any]:
         """Return dictionary of point values at `idx` for current chain
         with variable names as keys.
         """
diff --git a/pymc3/exceptions.py b/pymc3/exceptions.py
index 62d275e2b9..b2ff9f0c52 100644
--- a/pymc3/exceptions.py
+++ b/pymc3/exceptions.py
@@ -1,4 +1,4 @@
-__all__ = ['SamplingError', 'IncorrectArgumentsError']
+__all__ = ['SamplingError', 'IncorrectArgumentsError', 'TraceDirectoryError']
 
 
 class SamplingError(RuntimeError):
@@ -7,3 +7,7 @@ class SamplingError(RuntimeError):
 
 class IncorrectArgumentsError(ValueError):
     pass
+
+class TraceDirectoryError(ValueError):
+    '''Error from trying to load a trace from an incorrectly-structured directory,'''
+    pass
diff --git a/pymc3/tests/test_ndarray_backend.py b/pymc3/tests/test_ndarray_backend.py
index be438c706b..b5ea2daa80 100644
--- a/pymc3/tests/test_ndarray_backend.py
+++ b/pymc3/tests/test_ndarray_backend.py
@@ -213,6 +213,11 @@ def test_save_and_load(self, tmpdir_factory):
         for var in ('x', 'z'):
             assert (self.trace[var] == trace2[var]).all()
 
+    def test_bad_load(self, tmpdir_factory):
+        directory = str(tmpdir_factory.mktemp('data'))
+        with pytest.raises(pm.TraceDirectoryError):
+            pm.load_trace(directory, model=TestSaveLoad.model())
+
     def test_sample_posterior_predictive(self, tmpdir_factory):
         directory = str(tmpdir_factory.mktemp('data'))
         save_dir = pm.save_trace(self.trace, directory, overwrite=True)