diff --git a/Library/Homebrew/fetch.rb b/Library/Homebrew/fetch.rb
index edb17c0e05..3aeaee8929 100644
--- a/Library/Homebrew/fetch.rb
+++ b/Library/Homebrew/fetch.rb
@@ -18,7 +18,12 @@ module Homebrew
       bottle = formula.bottle
 
       return true if force_bottle && bottle.present?
-      return true if os.present?
+      if os.present?
+        return true
+      elsif ENV["HOMEBREW_TEST_GENERIC_OS"].present?
+        # `:generic` bottles don't exist, and `--os` flag is not specified.
+        return false
+      end
       return true if arch.present?
       return true if bottle_tag.present? && formula.bottled?(bottle_tag)
 
diff --git a/Library/Homebrew/simulate_system.rb b/Library/Homebrew/simulate_system.rb
index 2b1b0faa01..e4f1a75088 100644
--- a/Library/Homebrew/simulate_system.rb
+++ b/Library/Homebrew/simulate_system.rb
@@ -23,8 +23,8 @@ module Homebrew
         old_arch = self.arch
 
         begin
-          self.os = os if os
-          self.arch = arch if arch
+          self.os = os if os && os != current_os
+          self.arch = arch if arch && arch != current_arch
 
           yield
         ensure