diff --git a/docs/sophia_stdlib.md b/docs/sophia_stdlib.md
index 8b7138b..219b27c 100644
--- a/docs/sophia_stdlib.md
+++ b/docs/sophia_stdlib.md
@@ -1216,7 +1216,7 @@ List.unzip(l : list('a * 'b)) : list('a) * list('b)
 Opposite to the `zip` operation. Takes a list of pairs and returns pair of lists with respective elements on same indices.
 
 
-### merge
+#### merge
 ```
 List.merge(lesser_cmp : ('a, 'a) => bool, l1 : list('a), l2 : list('a)) : list('a)
 ```
@@ -1224,7 +1224,7 @@ List.merge(lesser_cmp : ('a, 'a) => bool, l1 : list('a), l2 : list('a)) : list('
 Merges two sorted lists into a single sorted list. O(length(l1) + length(l2))
 
 
-### sort
+#### sort
 ```
 List.sort(lesser_cmp : ('a, 'a) => bool, l : list('a)) : list('a)
 ```
diff --git a/priv/stdlib/List.aes b/priv/stdlib/List.aes
index 9493037..cf9670e 100644
--- a/priv/stdlib/List.aes
+++ b/priv/stdlib/List.aes
@@ -80,6 +80,7 @@ namespace List =
  * `b` only if `(b - a) mod step == 0`. `step` should be bigger than 0.
  */
   function from_to_step(a : int, b : int, s : int) : list(int) =
+    require(s > 0, "List.from_to_step: non-positive step")
     from_to_step_(a, b - (b-a) mod s, s, [])
   private function from_to_step_(a : int, b : int, s : int, acc : list(int)) : list(int) =
     if(b < a) acc