diff --git a/skimage/exposure/exposure.py b/skimage/exposure/exposure.py
index 8a945de1320..dc2faebdca6 100644
--- a/skimage/exposure/exposure.py
+++ b/skimage/exposure/exposure.py
@@ -501,7 +501,7 @@ def is_low_contrast(image, fraction_threshold=0.05, lower_percentile=1,
     False
     """
     image = np.asanyarray(image)
-    if image.ndim == 3 and image.shape[3] in [3, 4]:
+    if image.ndim == 3 and image.shape[2] in [3, 4]:
         image = rgb2gray(image)
 
     dlimits = dtype_limits(image)