diff --git a/hexrd/imageseries/save.py b/hexrd/imageseries/save.py
index 59e223346..0de243f51 100644
--- a/hexrd/imageseries/save.py
+++ b/hexrd/imageseries/save.py
@@ -195,11 +195,16 @@ def __init__(self, ims, fname, **kwargs):
         cache_file: str or Path, optional
            name of the npz file to save the image data, if not given in the
            `fname` argument; for YAML format (deprecated), this is required
+        max_workers: int, optional
+           The max number of worker threads for multithreading. Defaults to
+           the number of CPUs.
         """
         Writer.__init__(self, ims, fname, **kwargs)
         self._thresh = self._opts['threshold']
         self._cache, self.cachename = self._set_cache()
-        self.max_workers = kwargs.get('max_workers', None)
+
+        ncpus = multiprocessing.cpu_count()
+        self.max_workers = kwargs.get('max_workers', ncpus)
 
     def _set_cache(self):
 
@@ -254,9 +259,7 @@ def _write_frames(self):
         buff_size = self._ims.shape[0]*self._ims.shape[1]
         arrd = {}
 
-        ncpus = multiprocessing.cpu_count()
-        max_workers = ncpus if self.max_workers is None else self.max_workers
-        num_workers = min(max_workers, len(self._ims))
+        num_workers = min(self.max_workers, len(self._ims))
 
         row_buffers = np.empty((num_workers, buff_size), dtype=np.uint16)
         col_buffers = np.empty((num_workers, buff_size), dtype=np.uint16)
@@ -274,9 +277,6 @@ def extract_data(i):
             cols = col_buffers[buffer_id]
             vals = val_buffers[buffer_id]
 
-            # FIXME: in __init__() of ProcessedImageSeries:
-            # 'ProcessedImageSeries' object has no attribute '_adapter'
-
             # wrapper to find (sparse) pixels above threshold
             count = extract_ijv(self._ims[i], self._thresh,
                                 rows, cols, vals)
@@ -302,7 +302,9 @@ def extract_data(i):
             'initializer': assign_buffer_id,
         }
         with ThreadPoolExecutor(**kwargs) as executor:
-            executor.map(extract_data, range(len(self._ims)))
+            # Evaluate the results via `list()`, so that if an exception is
+            # raised in a thread, it will be re-raised and visible to the user.
+            list(executor.map(extract_data, range(len(self._ims))))
 
         arrd['shape'] = self._ims.shape
         arrd['nframes'] = len(self._ims)
diff --git a/hexrd/instrument/hedm_instrument.py b/hexrd/instrument/hedm_instrument.py
index 8f5b7e1ca..46350f5df 100644
--- a/hexrd/instrument/hedm_instrument.py
+++ b/hexrd/instrument/hedm_instrument.py
@@ -1257,7 +1257,10 @@ def extract_polar_maps(self, plane_data, imgser_dict,
                     func(task)
             else:
                 with ThreadPoolExecutor(max_workers=max_workers) as executor:
-                    executor.map(func, tasks)
+                    # Evaluate the results via `list()`, so that if an
+                    # exception is raised in a thread, it will be re-raised
+                    # and visible to the user.
+                    list(executor.map(func, tasks))
 
             ring_maps_panel[det_key] = ring_maps