diff --git a/gridlock/draw.py b/gridlock/draw.py
index 5390871..b2a3245 100644
--- a/gridlock/draw.py
+++ b/gridlock/draw.py
@@ -70,10 +70,13 @@ def draw_polygons(
                             + 'xyz'[surface_normal])
 
     # Broadcast foreground where necessary
-    if numpy.size(foreground) == 1:
-        foreground = [foreground] * len(cell_data)
+    foregrounds: Union[Sequence[foreground_callable_t], Sequence[float]]
+    if numpy.size(foreground) == 1:     # type: ignore
+        foregrounds = [foreground] * len(cell_data) # type: ignore
     elif isinstance(foreground, numpy.ndarray):
         raise GridError('ndarray not supported for foreground')
+    else:
+        foregrounds = foreground    # type: ignore
 
     # ## Compute sub-domain of the grid occupied by polygons
     # 1) Compute outer bounds (bd) of polygons
@@ -105,22 +108,23 @@ def draw_polygons(
         return numpy.insert(v_2d, surface_normal, (val,))
 
     # iterate over grids
-    for i, grid in enumerate(cell_data):
-        # ## Evaluate or expand foreground[i]
-        if callable(foreground[i]):
+    for i, _ in enumerate(cell_data):
+        # ## Evaluate or expand foregrounds[i]
+        foregrounds_i = foregrounds[i]
+        if callable(foregrounds_i):
             # meshgrid over the (shifted) domain
             domain = [self.shifted_xyz(i)[k][bdi_min[k]:bdi_max[k]+1] for k in range(3)]
             (x0, y0, z0) = numpy.meshgrid(*domain, indexing='ij')
 
             # evaluate on the meshgrid
-            foreground_i = foreground[i](x0, y0, z0)
-            if not numpy.isfinite(foreground_i).all():
+            foreground_val = foregrounds_i(x0, y0, z0)
+            if not numpy.isfinite(foreground_val).all():
                 raise GridError(f'Non-finite values in foreground[{i}]')
-        elif numpy.size(foreground[i]) != 1:
-            raise GridError(f'Unsupported foreground[{i}]: {type(foreground[i])}')
+        elif numpy.size(foregrounds_i) != 1:
+            raise GridError(f'Unsupported foreground[{i}]: {type(foregrounds_i)}')
         else:
             # foreground[i] is scalar non-callable
-            foreground_i = foreground[i]
+            foreground_val = foregrounds_i
 
         w_xy = numpy.zeros((bdi_max - bdi_min + 1)[surface].astype(int))
 
@@ -188,7 +192,7 @@ def draw_polygons(
 
         # ## Modify the grid
         g_slice = (i,) + tuple(numpy.s_[bdi_min[a]:bdi_max[a] + 1] for a in range(3))
-        cell_data[g_slice] = (1 - w) * cell_data[g_slice] + w * foreground_i
+        cell_data[g_slice] = (1 - w) * cell_data[g_slice] + w * foreground_val
 
 
 def draw_polygon(