diff --git a/mrbgems/mruby-bin-debugger/mrbgem.rake b/mrbgems/mruby-bin-debugger/mrbgem.rake
index 091851dd4..f51c9a3db 100644
--- a/mrbgems/mruby-bin-debugger/mrbgem.rake
+++ b/mrbgems/mruby-bin-debugger/mrbgem.rake
@@ -2,7 +2,7 @@ MRuby::Gem::Specification.new('mruby-bin-debugger') do |spec|
   spec.license = 'MIT'
   spec.author  = 'mruby developers'
   spec.summary = 'mruby debugger command'
-  spec.build.defines |= %w(MRB_USE_DEBUG_HOOK)
+  spec.build.defines << "MRB_USE_DEBUG_HOOK"
   spec.add_dependency('mruby-eval', :core => 'mruby-eval')
 
   spec.bins = %w(mrdb)