[merge] Improve assert
diff --git a/Lib/fontTools/merge.py b/Lib/fontTools/merge.py
index 3e5acec..48aee3c 100644
--- a/Lib/fontTools/merge.py
+++ b/Lib/fontTools/merge.py
@@ -33,9 +33,10 @@
 # General utility functions for merging values from different fonts
 
 def equal(lst):
+	lst = list(lst)
 	t = iter(lst)
 	first = next(t)
-	assert all(item == first for item in t)
+	assert all(item == first for item in t), "Expected all items to be equal: %s" % lst
 	return first
 
 def first(lst):