diff --git a/src/eynollah/utils/pil_cv2.py b/src/eynollah/utils/pil_cv2.py
index 7975939..36228a7 100644
--- a/src/eynollah/utils/pil_cv2.py
+++ b/src/eynollah/utils/pil_cv2.py
@@ -7,12 +7,14 @@ from cv2 import COLOR_GRAY2BGR, COLOR_RGB2BGR, COLOR_BGR2RGB, cvtColor, imread
 # from sbb_binarization
 
 def cv2pil(img):
-    return Image.fromarray(np.array(cvtColor(img, COLOR_BGR2RGB)))
+    # reduce depth because cvtColor is limited
+    return Image.fromarray(np.array(cvtColor(img.astype(np.uint8), COLOR_BGR2RGB)))
 
 def pil2cv(img):
     # from ocrd/workspace.py
     color_conversion = COLOR_GRAY2BGR if img.mode in ('1', 'L', 'LA') else  COLOR_RGB2BGR
     pil_as_np_array = np.array(img).astype('uint8') if img.mode == '1' else np.array(img)
+    # cvtColor cannot handle alpha
     if pil_as_np_array.shape[-1] == 2:
         pil_as_np_array = pil_as_np_array[:,:,0]
     elif pil_as_np_array.shape[-1] == 4: