diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d68a852f..851f7bcf 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -47,7 +47,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v1
         with:
-          python-version: "3.x"
+          python-version: "3.8"
 
       - name: Install dependencies
         run: |
diff --git a/CHANGES.md b/CHANGES.md
index 8e44acaa..0f0b067b 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,8 @@
+## 2.0.0b18 (2020-10-22)
+
+* surface dataset.nodata in COGReader.nodata property (https://github.com/cogeotiff/rio-tiler/pull/292)
+* fix non-threaded tasks scheduler/filter (https://github.com/cogeotiff/rio-tiler/pull/291)
+
 ## 2.0.0b17 (2020-10-13)
 
 * switch to morecantile for TMS definition (ref: https://github.com/cogeotiff/rio-tiler/issues/283)
diff --git a/rio_tiler/mosaic/reader.py b/rio_tiler/mosaic/reader.py
index 344d9006..63d4a92c 100644
--- a/rio_tiler/mosaic/reader.py
+++ b/rio_tiler/mosaic/reader.py
@@ -73,7 +73,7 @@ def reader(asset: str, *args, **kwargs) -> Tuple[numpy.ndarray, numpy.ndarray]:
         )
 
     if not chunk_size:
-        chunk_size = threads or len(assets)
+        chunk_size = threads if threads > 1 else len(assets)
 
     assets_used: List[str] = []
 
diff --git a/rio_tiler/tasks.py b/rio_tiler/tasks.py
index 6bc360fc..b92cb511 100644
--- a/rio_tiler/tasks.py
+++ b/rio_tiler/tasks.py
@@ -1,26 +1,15 @@
 """rio_tiler.tasks: tools for handling rio-tiler's future tasks."""
 
 from concurrent import futures
-from typing import (
-    Any,
-    Callable,
-    Dict,
-    Generator,
-    Iterator,
-    Optional,
-    Sequence,
-    Tuple,
-    Union,
-)
+from functools import partial
+from typing import Any, Callable, Dict, Generator, Optional, Sequence, Tuple, Union
 
 import numpy
 
 from .constants import MAX_THREADS
 from .logger import logger
 
-TaskType = Union[
-    Generator[Tuple[Callable, str], None, None], Iterator[Tuple[futures.Future, str]],
-]
+TaskType = Sequence[Tuple[Union[futures.Future, Callable], str]]
 
 
 def filter_tasks(
@@ -31,8 +20,8 @@ def filter_tasks(
 
     Attributes
     ----------
-    tasks: list or generator
-        Sequence of 'concurrent.futures._base.Future' or 'callable'
+    tasks: list
+        Sequence of 'concurrent.futures._base.Future' or 'Callable'
     allowed_exceptions: Tuple, optional
         List of exceptions which won't be raised.
 
@@ -44,15 +33,12 @@ def filter_tasks(
     if not allowed_exceptions:
         allowed_exceptions = ()
 
-    while True:
+    for (future, asset) in tasks:
         try:
-            future, asset = next(tasks)  # type: ignore
             if isinstance(future, futures.Future):
                 yield future.result(), asset
             else:
-                yield future, asset
-        except StopIteration:
-            break
+                yield future(), asset
         except allowed_exceptions as err:
             logger.info(err)
             pass
@@ -61,15 +47,15 @@ def filter_tasks(
 def create_tasks(reader: Callable, assets, threads, *args, **kwargs) -> TaskType:
     """Create Future Tasks."""
     if threads and threads > 1:
+        logger.debug(f"Running tasks in ThreadPool with max_workers={threads}")
         with futures.ThreadPoolExecutor(max_workers=threads) as executor:
-            return iter(
-                [
-                    (executor.submit(reader, asset, *args, **kwargs), asset)
-                    for asset in assets
-                ]
-            )
+            return [
+                (executor.submit(reader, asset, *args, **kwargs), asset)
+                for asset in assets
+            ]
     else:
-        return ((reader(asset, *args, **kwargs), asset) for asset in assets)
+        logger.debug(f"Running tasks outside ThreadsPool (max_workers={threads})")
+        return [(partial(reader, asset, *args, **kwargs), asset) for asset in assets]
 
 
 def multi_arrays(
diff --git a/tests/test_mosaic.py b/tests/test_mosaic.py
index d988ca86..eccf358c 100644
--- a/tests/test_mosaic.py
+++ b/tests/test_mosaic.py
@@ -35,8 +35,7 @@
 def _read_tile(src_path: str, *args, **kwargs) -> Tuple[numpy.ndarray, numpy.ndarray]:
     """Read tile from an asset"""
     with COGReader(src_path) as cog:
-        tile, mask = cog.tile(*args, **kwargs)
-    return tile, mask
+        return cog.tile(*args, **kwargs)
 
 
 def _read_preview(
@@ -222,18 +221,64 @@ def test_mosaic_tiler_Stdev():
 
 def test_threads():
     """Test mosaic tiler."""
-    assets = [asset1, asset2, asset1, asset2, asset1, asset2]
+    assets = [asset2, asset1, asset1, asset2, asset1, asset2]
 
     # TileOutSide bounds should be ignore and thus Tile is None
     (tnothread, _), _ = mosaic.mosaic_reader(assets, _read_tile, xo, yo, zo, threads=2)
     assert not tnothread
 
+    # TileOutSide bounds should be ignore and thus Tile is None
     (tnothread, _), _ = mosaic.mosaic_reader(assets, _read_tile, xo, yo, zo, threads=0)
     assert not tnothread
 
-    (tnothread, _), _ = mosaic.mosaic_reader(assets, _read_tile, x, y, z, threads=0)
+    # Only cover asset1
+    xpp = 147
+    ypp = 180
+    zpp = 9
+
+    # Partial tile, some assets should Error with TileOutside bounds
+    (tnothread, _), a = mosaic.mosaic_reader(
+        assets,
+        _read_tile,
+        xpp,
+        ypp,
+        zpp,
+        threads=0,
+        pixel_selection=defaults.MedianMethod,
+    )
+    assert len(a) == 3
+    assert tnothread.shape
+
+    # Partial tile, some assets should Error with TileOutside bounds
+    (tnothread, _), a = mosaic.mosaic_reader(
+        assets,
+        _read_tile,
+        xpp,
+        ypp,
+        zpp,
+        threads=1,
+        pixel_selection=defaults.MedianMethod,
+    )
+    assert len(a) == 3
+    assert tnothread.shape
+
+    (tnothread, _), _ = mosaic.mosaic_reader(
+        assets,
+        _read_tile,
+        xpp,
+        ypp,
+        zpp,
+        threads=0,
+        pixel_selection=defaults.MedianMethod,
+    )
     (tmulti_threads, _), _ = mosaic.mosaic_reader(
-        assets, _read_tile, x, y, z, threads=1
+        assets,
+        _read_tile,
+        xpp,
+        ypp,
+        zpp,
+        threads=3,
+        pixel_selection=defaults.MedianMethod,
     )
     numpy.testing.assert_array_equal(tnothread, tmulti_threads)