diff --git a/build_config/ci/gcc-clang.rb b/build_config/ci/gcc-clang.rb
index eeaab5d0f..f6303e49e 100644
--- a/build_config/ci/gcc-clang.rb
+++ b/build_config/ci/gcc-clang.rb
@@ -6,7 +6,7 @@ MRuby::Build.new('full-debug') do |conf|
 
   # include all core GEMs
   conf.gembox 'full-core'
-  conf.cc.defines += %w(MRB_GC_STRESS MRB_USE_DEBUG_HOOK MRB_UTF8_STRING)
+  conf.cc.defines += %w(MRB_GC_STRESS MRB_USE_DEBUG_HOOK)
 
   conf.enable_test
 end
@@ -18,7 +18,7 @@ MRuby::Build.new do |conf|
   conf.gembox 'full-core'
   conf.gem :core => 'mruby-bin-debugger'
   conf.compilers.each do |c|
-    c.defines += %w(MRB_GC_FIXED_ARENA MRB_UTF8_STRING)
+    c.defines += %w(MRB_GC_FIXED_ARENA)
   end
   conf.enable_bintest
   conf.enable_test
@@ -30,7 +30,7 @@ MRuby::Build.new('cxx_abi') do |conf|
   conf.gembox 'full-core'
   conf.cc.flags += %w(-fpermissive -std=gnu++03)
   conf.compilers.each do |c|
-    c.defines += %w(MRB_GC_FIXED_ARENA MRB_UTF8_STRING)
+    c.defines += %w(MRB_GC_FIXED_ARENA)
   end
   conf.enable_test
 
diff --git a/build_config/host-debug.rb b/build_config/host-debug.rb
index 66e622ad6..600a4d78f 100644
--- a/build_config/host-debug.rb
+++ b/build_config/host-debug.rb
@@ -8,7 +8,7 @@ MRuby::Build.new('host') do |conf|
   conf.gembox 'full-core'
 
   # C compiler settings
-  conf.cc.defines = %w(MRB_USE_DEBUG_HOOK MRB_NO_BOXING MRB_UTF8_STRING)
+  conf.cc.defines = %w(MRB_USE_DEBUG_HOOK MRB_NO_BOXING)
 
   # Generate mruby debugger command (require mruby-eval)
   conf.gem :core => "mruby-bin-debugger"