diff --git a/README.md b/README.md
index c375a059..c46068cc 100644
--- a/README.md
+++ b/README.md
@@ -6575,6 +6575,8 @@ The optional function 'func' is a predicate that takes as argument an object of
 container and returns a boolean that is true if the object has to be added
 to the other container.
 
+Both containers shall either provide PUSH method, or SET\_KEY method.
+
 
 #### ALGO\_REDUCE(dest, container, oplist, reduceFunc[, mapFunc[, arguments..])