diff --git a/test/gdbm/test_gdbm.rb b/test/gdbm/test_gdbm.rb
index 64692a4465..e02282b451 100644
--- a/test/gdbm/test_gdbm.rb
+++ b/test/gdbm/test_gdbm.rb
@@ -143,7 +143,7 @@ if defined? GDBM
 
     def open_db_child(dbname, *opts)
       opts = [0644, *opts].map(&:inspect).join(', ')
-      args = [EnvUtil.rubybin, "-rgdbm", "-e", <<-SRC, dbname]
+      args = [EnvUtil.rubybin, "-Ilib", "-rgdbm", "-e", <<-SRC, dbname]
       STDOUT.sync = true
       gdbm = GDBM.open(ARGV.shift, #{opts})
       puts gdbm.class