diff --git a/src/ocrmypdf/leptonica.py b/src/ocrmypdf/leptonica.py
index 6d382795..ab26f750 100644
--- a/src/ocrmypdf/leptonica.py
+++ b/src/ocrmypdf/leptonica.py
@@ -139,6 +139,8 @@ class Pix:
     """
 
     def __init__(self, pix):
+        if not pix:
+            raise ValueError('NULL box')
         self._pix = ffi.gc(pix, Pix._pix_destroy)
 
     def __repr__(self):
@@ -403,8 +405,6 @@ class Pix:
                 display,
                 pdfdir))
 
-            print(repr(cropbox))
-
             cropped_pix = lept.pixClipRectangle(
                 self._pix,
                 cropbox._box,
@@ -549,8 +549,11 @@ class Box:
     """
 
     def __init__(self, box):
+        if not box:
+            raise ValueError('NULL box')
         self._box = ffi.gc(box, Box._box_destroy)
 
+
     def __repr__(self):
         if self._box:
             return '<leptonica.Box x={0} y={1} w={2} h={3}>'.format(
@@ -583,6 +586,8 @@ class BoxArray:
     """Wrapper around Leptonica's BOXA (Array of BOX) objects."""
 
     def __init__(self, boxa):
+        if not boxa:
+            raise ValueError('NULL boxa')
         self._boxa = ffi.gc(boxa, BoxArray._boxa_destroy)
 
     def __repr__(self):
diff --git a/tests/test_lept.py b/tests/test_lept.py
index 9c03a6b8..38efebc2 100644
--- a/tests/test_lept.py
+++ b/tests/test_lept.py
@@ -18,8 +18,12 @@
 
 import os
 import shutil
-import pytest
 import sys
+from pickle import dumps, loads
+
+import pytest
+from PIL import Image, ImageChops
+
 import ocrmypdf.leptonica as lept
 
 
@@ -28,3 +32,49 @@ def test_colormap_backgroundnorm(resources):
     # can handle that case
     pix = lept.Pix.open(resources / 'baiona_colormapped.png')
     pix.background_norm()
+
+
+@pytest.fixture
+def crom_pix(resources):
+    pix = lept.Pix.open(resources / 'crom.png')
+    im = Image.open(resources / 'crom.png')
+    return pix, im
+
+
+def test_pix_basic(crom_pix):
+    pix, im = crom_pix
+
+    assert pix.width == im.width
+    assert pix.height == im.height
+    assert pix.mode == im.mode
+
+
+def test_pil_conversion(crom_pix):
+    pix, im = crom_pix
+
+    # Check for pixel perfect
+    assert ImageChops.difference(pix.topil(), im).getbbox() is None
+
+
+def test_pix_otsu(crom_pix):
+    pix, _ = crom_pix
+    im1bpp = pix.otsu_adaptive_threshold()
+    assert im1bpp.mode == '1'
+
+
+def test_crop(resources):
+    pix = lept.Pix.open(resources / 'linn.png')
+    foreground = pix.crop_to_foreground()
+    assert foreground.width < pix.width
+
+
+def test_clean_bg(crom_pix):
+    pix, _ = crom_pix
+    imbg = pix.clean_background_to_white()
+
+
+def test_pickle(crom_pix):
+    pix, _ = crom_pix
+    pickled = dumps(pix)
+    pix2 = loads(pickled)
+    assert pix.mode == pix2.mode