diff --git a/lib/http/request/body.rb b/lib/http/request/body.rb
index 14af30b8..46ce4f24 100644
--- a/lib/http/request/body.rb
+++ b/lib/http/request/body.rb
@@ -24,7 +24,8 @@ def size
         elsif @source.nil?
           0
         else
-          raise RequestError, "cannot determine size of body: #{@source.inspect}"
+          raise RequestError, "cannot determine size of body: #{@source.inspect}, " \
+            "please set 'Content-Length' header explicitly"
         end
       end