diff --git a/yirgacheffe/layers/area.py b/yirgacheffe/layers/area.py
index 5d1f7c9..23d8ed4 100644
--- a/yirgacheffe/layers/area.py
+++ b/yirgacheffe/layers/area.py
@@ -65,10 +65,13 @@ def __init__(self, dataset, name: Optional[str] = None, band: int = 1):
 
         transform = dataset.GetGeoTransform()
 
+        pixel_scale = self.pixel_scale
+        assert pixel_scale # from raster we should always have one
+
         self._underlying_area = Area(
-            floor(-180 / self.pixel_scale.xstep) * self.pixel_scale.xstep,
+            floor(-180 / pixel_scale.xstep) * pixel_scale.xstep,
             self.area.top,
-            ceil(180 / self.pixel_scale.xstep) * self.pixel_scale.xstep,
+            ceil(180 / pixel_scale.xstep) * pixel_scale.xstep,
             self.area.bottom
         )
         self._active_area = self._underlying_area
diff --git a/yirgacheffe/layers/group.py b/yirgacheffe/layers/group.py
index ecec231..86aae8e 100644
--- a/yirgacheffe/layers/group.py
+++ b/yirgacheffe/layers/group.py
@@ -41,7 +41,7 @@ def layer_from_files(cls, filenames: List[str], name: Optional[str] = None) -> G
         rasters = [RasterLayer.layer_from_file(x) for x in filenames]
         return cls(rasters, name)
 
-    def __init__(self, layers: List[YirgacheffeLayer], name: Optional[str] = None) -> GroupLayerT:
+    def __init__(self, layers: List[YirgacheffeLayer], name: Optional[str] = None) -> None:
         if not layers:
             raise GroupLayerEmpty("Expected one or more layers")
         if not are_pixel_scales_equal_enough([x.pixel_scale for x in layers]):
diff --git a/yirgacheffe/layers/rasters.py b/yirgacheffe/layers/rasters.py
index 1e2d3e0..13e31b5 100644
--- a/yirgacheffe/layers/rasters.py
+++ b/yirgacheffe/layers/rasters.py
@@ -279,11 +279,14 @@ def _unpark(self):
     def datatype(self) -> int:
         if self._dataset is None:
             self._unpark()
+        assert self._dataset
         return self._dataset.GetRasterBand(1).DataType
 
     def read_array(self, xoffset, yoffset, xsize, ysize) -> Any:
         if self._dataset is None:
             self._unpark()
+        assert self._dataset
+
         if (xsize <= 0) or (ysize <= 0):
             raise ValueError("Request dimensions must be positive and non-zero")
 
diff --git a/yirgacheffe/layers/rescaled.py b/yirgacheffe/layers/rescaled.py
index 480b526..610f5e6 100644
--- a/yirgacheffe/layers/rescaled.py
+++ b/yirgacheffe/layers/rescaled.py
@@ -39,8 +39,11 @@ def __init__(
         self._src = src
         self._nearest_neighbour = nearest_neighbour
 
-        self._x_scale = src._pixel_scale.xstep / pixel_scale.xstep
-        self._y_scale = src._pixel_scale.ystep / pixel_scale.ystep
+        src_pixel_scale = src.pixel_scale
+        assert src_pixel_scale # from raster we should always have one
+
+        self._x_scale = src_pixel_scale.xstep / pixel_scale.xstep
+        self._y_scale = src_pixel_scale.ystep / pixel_scale.ystep
 
     def close(self):
         self._src.close()