diff --git a/lib/slayer/command.rb b/lib/slayer/command.rb
index a3cf29b..c25737b 100644
--- a/lib/slayer/command.rb
+++ b/lib/slayer/command.rb
@@ -22,7 +22,7 @@ def err(value: nil, status: :default, message: nil)
       end
 
       def err!(value: nil, status: :default, message: nil)
-        unless ENV[:SUPPRESS_SLAYER_WARNINGS]
+        unless ENV['SUPPRESS_SLAYER_WARNINGS']
           warn '[DEPRECATION] `err!` is deprecated.  Please use `return err` instead.'
         end
         raise ResultFailureError, err(value: value, status: status, message: message)
diff --git a/lib/slayer/compat/compat_040.rb b/lib/slayer/compat/compat_040.rb
index c7fb571..9d81bc6 100644
--- a/lib/slayer/compat/compat_040.rb
+++ b/lib/slayer/compat/compat_040.rb
@@ -4,17 +4,17 @@ module Slayer
   class Command
     class << self
       def pass(value: nil, status: :default, message: nil)
-        warn '[DEPRECATION] `pass` is deprecated.  Please use `ok` instead.' unless ENV[:SUPPRESS_SLAYER_WARNINGS]
+        warn '[DEPRECATION] `pass` is deprecated.  Please use `ok` instead.' unless ENV['SUPPRESS_SLAYER_WARNINGS']
         ok(value: value, status: status, message: message)
       end
 
       def flunk(value: nil, status: :default, message: nil)
-        warn '[DEPRECATION] `flunk` is deprecated.  Please use `err` instead.' unless ENV[:SUPPRESS_SLAYER_WARNINGS]
+        warn '[DEPRECATION] `flunk` is deprecated.  Please use `err` instead.' unless ENV['SUPPRESS_SLAYER_WARNINGS']
         err(value: value, status: status, message: message)
       end
 
       def flunk!(value: nil, status: :default, message: nil)
-        unless ENV[:SUPPRESS_SLAYER_WARNINGS]
+        unless ENV['SUPPRESS_SLAYER_WARNINGS']
           warn '[DEPRECATION] `flunk!` is deprecated.  Please use `return err` instead.'
         end
         err!(value: value, status: status, message: message)
@@ -28,24 +28,24 @@ def flunk!(value: nil, status: :default, message: nil)
 
   class Result
     def success?
-      warn '[DEPRECATION] `success?` is deprecated.  Please use `ok?` instead.' unless ENV[:SUPPRESS_SLAYER_WARNINGS]
+      warn '[DEPRECATION] `success?` is deprecated.  Please use `ok?` instead.' unless ENV['SUPPRESS_SLAYER_WARNINGS']
       ok?
     end
 
     def failure?
-      warn '[DEPRECATION] `failure?` is deprecated.  Please use `err?` instead.' unless ENV[:SUPPRESS_SLAYER_WARNINGS]
+      warn '[DEPRECATION] `failure?` is deprecated.  Please use `err?` instead.' unless ENV['SUPPRESS_SLAYER_WARNINGS']
       err?
     end
   end
 
   class ResultMatcher
     def pass(...)
-      warn '[DEPRECATION] `pass` is deprecated.  Please use `ok` instead.' unless ENV[:SUPPRESS_SLAYER_WARNINGS]
+      warn '[DEPRECATION] `pass` is deprecated.  Please use `ok` instead.' unless ENV['SUPPRESS_SLAYER_WARNINGS']
       ok(...)
     end
 
     def fail(...)
-      warn '[DEPRECATION] `fail` is deprecated.  Please use `err` instead.' unless ENV[:SUPPRESS_SLAYER_WARNINGS]
+      warn '[DEPRECATION] `fail` is deprecated.  Please use `err` instead.' unless ENV['SUPPRESS_SLAYER_WARNINGS']
       err(...)
     end
   end