diff --git a/.gitignore b/.gitignore
index f8cecb9c4..76861bb04 100644
--- a/.gitignore
+++ b/.gitignore
@@ -24,7 +24,7 @@ benchmark/**/*.dat
 benchmark/*.pdf
 benchmark/*.png
 bin
-build
+/build
 doc/api
 doc/capi
 compile_commands.json
diff --git a/Rakefile b/Rakefile
index 79ec84f67..4422f9688 100644
--- a/Rakefile
+++ b/Rakefile
@@ -4,6 +4,7 @@ MRUBY_ROOT = File.dirname(File.expand_path(__FILE__))
 MRUBY_BUILD_HOST_IS_CYGWIN = RUBY_PLATFORM.include?('cygwin')
 MRUBY_BUILD_HOST_IS_OPENBSD = RUBY_PLATFORM.include?('openbsd')
 
+Rake.application.options.always_multitask = true unless ENV["SERIAL"]
 Rake.verbose(false) if Rake.verbose == Rake::DSL::DEFAULT
 
 $LOAD_PATH << File.join(MRUBY_ROOT, "lib")
diff --git a/doc/guides/mrbgems.md b/doc/guides/mrbgems.md
index 7087965e7..5c63ffb71 100644
--- a/doc/guides/mrbgems.md
+++ b/doc/guides/mrbgems.md
@@ -256,7 +256,7 @@ When more than one version requirements is passed, the dependency must satisfy a
 
 You can have default gem to use as dependency when it's not defined in your build configuration.
 When the last argument of `add_dependency` call is `Hash`, it will be treated as default gem information.
-Its format is same as argument of method `MRuby::Build#gem`, expect that it can't be treated as path gem location.
+Its format is same as argument of method `MRuby::Build#gem`, except that it can't be treated as path gem location.
 
 When a special version of dependency is required,
 use `MRuby::Build#gem` in the build configuration to override default gem.
@@ -304,10 +304,10 @@ Your GEM can export include paths to another GEMs that depends on your GEM.
 By default, `/...absolute path.../{GEM_NAME}/include` will be exported.
 So it is recommended not to put GEM's local header files on include/.
 
-These exports are retroactive.
+These exports are transitive.
 For example: when B depends on C and A depends on B, A will get include paths exported by C.
 
-Exported include_paths are automatically appended to GEM local include_paths by rake.
+Exported `include_paths` are automatically appended to GEM local `include_paths` by rake.
 You can use `spec.export_include_paths` accessor if you want more complex build.
 
 ## C Extension
diff --git a/mrbgems/mruby-pack/src/pack.c b/mrbgems/mruby-pack/src/pack.c
index 43b6e0f32..1b068a8fa 100644
--- a/mrbgems/mruby-pack/src/pack.c
+++ b/mrbgems/mruby-pack/src/pack.c
@@ -1534,8 +1534,8 @@ pack_uu(mrb_state *mrb, mrb_value src, mrb_value dst, mrb_int didx, int count)
 static int
 unpack_uu(mrb_state *mrb, const void *src, mrb_int slen, mrb_value ary)
 {
-  const char *s = (const char*)src;
-  const char *send = s + slen;
+  const unsigned char *s = (const unsigned char*)src;
+  const unsigned char *send = s + slen;
   mrb_value result = mrb_str_new(mrb, 0, slen * 3 / 4); /* estimate result size */
   char *dptr = RSTRING_PTR(result);
   char *dptr_start = dptr;