diff --git a/lib/set/sorted_set.rb b/lib/set/sorted_set.rb
index e4e01ba4a8..bc07bc1fb0 100644
--- a/lib/set/sorted_set.rb
+++ b/lib/set/sorted_set.rb
@@ -1,6 +1,6 @@
 begin
   require 'sorted_set'
 rescue ::LoadError
-  raise "The `SortedSet` class has been extracted from the `set` library." \
+  raise "The `SortedSet` class has been extracted from the `set` library. " \
         "You must use the `sorted_set` gem or other alternatives."
 end