diff --git a/doc/examples/plot_rag_merge.py b/doc/examples/plot_rag_merge.py
index 0f8ca1cb..d454a096 100644
--- a/doc/examples/plot_rag_merge.py
+++ b/doc/examples/plot_rag_merge.py
@@ -61,8 +61,10 @@ img = data.coffee()
 labels = segmentation.slic(img, compactness=30, n_segments=400)
 g = graph.rag_mean_color(img, labels)
 
-labels2 = graph.merge_hierarchical(labels, g, 40, False, True,
-                                   merge_mean_color, _weight_mean_color)
+labels2 = graph.merge_hierarchical(labels, g, thresh=40, rag_copy=False,
+                                   in_place_merge=True,
+                                   merge_func=merge_mean_color,
+                                   weight_func=_weight_mean_color)
 
 g2 = graph.rag_mean_color(img, labels2)
 
diff --git a/skimage/graph/tests/test_rag.py b/skimage/graph/tests/test_rag.py
index df78ed7d..ca21b27c 100644
--- a/skimage/graph/tests/test_rag.py
+++ b/skimage/graph/tests/test_rag.py
@@ -135,7 +135,7 @@ def test_rag_hierarchical():
     img = np.zeros((8, 8, 3), dtype='uint8')
     labels = np.zeros((8, 8), dtype='uint8')
 
-    img[:, :, :] = 30
+    img[:, :, :] = 31
     labels[:, :] = 1
 
     img[0:4, 0:4, :] = 10, 10, 10
@@ -146,14 +146,18 @@ def test_rag_hierarchical():
 
     g = graph.rag_mean_color(img, labels)
     g2 = g.copy()
-    thresh = 17.3206  # just above 10*sqrt(3)
+    thresh = 20  # more than 11*sqrt(3)
 
     result = merge_hierarchical_mean_color(labels, g, thresh)
-    assert len(np.unique(result)) == 2
+    assert(np.all(result[0:4, 0:4] == result[0, 0]))
+    assert(np.all(result[4:, 0:4] == result[4, 0]))
+    assert(np.all(result[:, 4:] == result[-1, -1]))
 
     result = merge_hierarchical_mean_color(labels, g2, thresh,
                                            in_place_merge=True)
-    assert len(np.unique(result)) == 2
+    assert(np.all(result[0:4, 0:4] == result[0, 0]))
+    assert(np.all(result[4:, 0:4] == result[4, 0]))
+    assert(np.all(result[:, 4:] == result[-1, -1]))
 
     result = graph.cut_threshold(labels, g, thresh)
-    assert len(np.unique(result)) == 1
+    assert np.all(result == result[0, 0])