diff --git a/Steepfile b/Steepfile
index 143e53ba..045ca1c3 100644
--- a/Steepfile
+++ b/Steepfile
@@ -6,7 +6,9 @@ target :lib do
   signature "ruby/testdata/example/sig"
   signature "_gem/sig"
 
-  check "ruby/testdata/example/lib" # Directory name
+  check "ruby/testdata/example/lib"
+  check "_gem/lib"
+
   # check "Gemfile"                   # File name
   # check "app/models/**/*.rb"        # Glob
   # ignore "lib/templates/*.rb"
diff --git a/_gem/go_gem.gemspec b/_gem/go_gem.gemspec
index ca92a1e4..a2197335 100644
--- a/_gem/go_gem.gemspec
+++ b/_gem/go_gem.gemspec
@@ -26,7 +26,7 @@ Gem::Specification.new do |spec|
   spec.files = IO.popen(%w[git ls-files -z], chdir: __dir__, err: IO::NULL) do |ls|
     ls.readlines("\x0", chomp: true).reject do |f|
       (f == gemspec) ||
-        f.start_with?(*%w[bin/ test/ spec/ features/ .git .github appveyor Gemfile])
+        f.start_with?(*%w[bin/ test/ spec/ features/ .git .github appveyor Gemfile sig/non-gemify/])
     end
   end
   spec.bindir = "exe"
diff --git a/_gem/lib/go_gem/mkmf.rb b/_gem/lib/go_gem/mkmf.rb
index 3b2ce97d..374676bf 100644
--- a/_gem/lib/go_gem/mkmf.rb
+++ b/_gem/lib/go_gem/mkmf.rb
@@ -26,7 +26,7 @@ def $objs.empty?; false; end
 
       create_makefile(target, srcprefix)
 
-      case `#{CONFIG["CC"]} --version` # rubocop:disable Lint/LiteralAsCondition
+      case `#{RbConfig::CONFIG["CC"]} --version` # rubocop:disable Lint/LiteralAsCondition
       when /Free Software Foundation/
         ldflags = "-Wl,--unresolved-symbols=ignore-all"
       when /clang/
diff --git a/_gem/sig/go_gem.rbs b/_gem/sig/go_gem.rbs
index 616aa22e..0a4b405c 100644
--- a/_gem/sig/go_gem.rbs
+++ b/_gem/sig/go_gem.rbs
@@ -1,4 +1,8 @@
 module GoGem
   VERSION: String
+
+  class Error < StandardError
+  end
+
   # See the writing guide of rbs: https://github.com/ruby/rbs#guides
 end
diff --git a/_gem/sig/go_gem/mkmf.rbs b/_gem/sig/go_gem/mkmf.rbs
index 79de1cb6..bbc2e696 100644
--- a/_gem/sig/go_gem/mkmf.rbs
+++ b/_gem/sig/go_gem/mkmf.rbs
@@ -1,5 +1,7 @@
 module GoGem
   module Mkmf
-    def create_go_makefile: (String, String? nil) -> void
+    $objs: Array[untyped]
+
+    def create_go_makefile: (String target, String? srcprefix) -> void
   end
 end
diff --git a/_gem/sig/non-gemify/mkmf.rbs b/_gem/sig/non-gemify/mkmf.rbs
new file mode 100644
index 00000000..ddaa08c8
--- /dev/null
+++ b/_gem/sig/non-gemify/mkmf.rbs
@@ -0,0 +1,7 @@
+module Kernel
+  def include: (*untyped) -> void
+
+  def find_executable: (String bin, ?String path) -> String?
+
+  def create_makefile: (String target, String? srcprefix) -> void
+end