Ractor.select requires an argument or yield_value

1 files changed, 2 insertions, 0 deletions

diff --git a/ractor.rb b/ractor.rb
index 89e39e5e1c..ca09aee7be 100644
--- a/ractor.rb
+++ b/ractor.rb
@@ -71,6 +71,8 @@ class Ractor
   #   #   and r is :yield
   #
   def self.select(*ractors, yield_value: yield_unspecified = true, move: false)
+    raise ArgumentError, 'specify at least one ractor or `yield_value`' if yield_unspecified && ractors.empty?
+
     __builtin_cstmt! %q{
       const VALUE *rs = RARRAY_CONST_PTR_TRANSIENT(ractors);
       VALUE rv;