diff --git a/skimage/filter/tests/test_thresholding.py b/skimage/filter/tests/test_thresholding.py
index 7d42fbcf..f0a68f7d 100644
--- a/skimage/filter/tests/test_thresholding.py
+++ b/skimage/filter/tests/test_thresholding.py
@@ -165,13 +165,13 @@ def test_isodata_moon_image():
 
 
 def test_isodata_moon_image_negative_int():
-    moon = data.moon().astype(np.int32)
+    moon = skimage.img_as_ubyte(data.moon()).astype(np.int32)
     moon -= 100
     assert threshold_isodata(moon) == -13
 
 
 def test_isodata_moon_image_negative_float():
-    moon = data.moon().astype(np.float64)
+    moon = skimage.img_as_ubyte(data.moon()).astype(np.float64)
     moon -= 100
     assert -13 < threshold_isodata(moon) < -12