diff --git a/lib/rb/lib/thrift/union.rb b/lib/rb/lib/thrift/union.rb
index 490c55c406..88c4285550 100644
--- a/lib/rb/lib/thrift/union.rb
+++ b/lib/rb/lib/thrift/union.rb
@@ -73,6 +73,8 @@ def write(oprot)
       fid = self.name_to_id(@setfield.to_s)
 
       field_info = struct_fields[fid]
+      raise "set_field is not valid for this union!" unless field_info
+
       type = field_info[:type]
       if is_container? type
         oprot.write_field_begin(@setfield, type, fid)