From 73a51400b682bd9cee981c18ed002fc81f8c2ce9 Mon Sep 17 00:00:00 2001
From: Mike J Innes <mike.j.innes@gmail.com>
Date: Mon, 30 Apr 2018 12:09:15 +0100
Subject: [PATCH] better error message

---
 src/tracker/array.jl | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/tracker/array.jl b/src/tracker/array.jl
index bb55ef73..89dcfac9 100644
--- a/src/tracker/array.jl
+++ b/src/tracker/array.jl
@@ -41,7 +41,7 @@ end
 Base.setindex!(xs::TrackedArray, v, i...) =
   error("Can't differentiate `setindex!`")
 
-back!(::TrackedArray) = error("Use back!(x, Δ)")
+back!(::TrackedArray) = error("Value is not scalar; use `back!(sum(x))` or `back!(x, Δ)`")
 
 # Fallthrough methods