diff --git a/Steepfile b/Steepfile
index 12efa39a..a8d034a8 100644
--- a/Steepfile
+++ b/Steepfile
@@ -4,4 +4,5 @@ target :lib do
   signature "sig"
 
   check "lib/lrama/bitmap.rb"
+  check "lib/lrama/warning.rb"
 end
diff --git a/sig/lrama/warning.rbs b/sig/lrama/warning.rbs
new file mode 100644
index 00000000..264f0be0
--- /dev/null
+++ b/sig/lrama/warning.rbs
@@ -0,0 +1,16 @@
+module Lrama
+  class Warning
+    interface _Appendable
+      def <<: (String message) -> self
+    end
+
+    @out: _Appendable
+
+    attr_reader errors: Array[String]
+    attr_reader warns: Array[String]
+    def initialize: (?_Appendable out) -> void
+    def error: (String message) -> void
+    def warn: (String message) -> void
+    def has_error?: -> bool
+  end
+end