diff --git a/Library/Homebrew/extend/on_system.rb b/Library/Homebrew/extend/on_system.rb
index 98703fb79f..5ec59cbdfa 100644
--- a/Library/Homebrew/extend/on_system.rb
+++ b/Library/Homebrew/extend/on_system.rb
@@ -93,7 +93,7 @@ module OnSystem
     base.define_method(:on_system) do |linux, macos:, &block|
       @on_system_blocks_exist = true
 
-      raise ArgumentError, "The first argument to `on_system` must be `:linux`" unless linux == :linux
+      raise ArgumentError, "The first argument to `on_system` must be `:linux`" if linux != :linux
 
       os_version, or_condition = if macos.to_s.include?("_or_")
         macos.to_s.split(/_(?=or_)/).map(&:to_sym)
diff --git a/Library/Homebrew/test/formula_spec.rb b/Library/Homebrew/test/formula_spec.rb
index a368fee1d0..e47a197361 100644
--- a/Library/Homebrew/test/formula_spec.rb
+++ b/Library/Homebrew/test/formula_spec.rb
@@ -1604,35 +1604,35 @@ describe Formula do
       end.new
     end
 
-    it "doesn't call code on Ventura" do
+    it "doesn't call code on Ventura", :needs_macos do
       Homebrew::SimulateSystem.os = :ventura
       f.brew { f.install }
       expect(f.foo).to eq(0)
       expect(f.bar).to eq(0)
     end
 
-    it "calls code on Linux" do
+    it "calls code on Linux", :needs_linux do
       Homebrew::SimulateSystem.os = :linux
       f.brew { f.install }
       expect(f.foo).to eq(1)
       expect(f.bar).to eq(1)
     end
 
-    it "calls code within `on_system :linux, macos: :monterey` on Monterey" do
+    it "calls code within `on_system :linux, macos: :monterey` on Monterey", :needs_macos do
       Homebrew::SimulateSystem.os = :monterey
       f.brew { f.install }
       expect(f.foo).to eq(1)
       expect(f.bar).to eq(0)
     end
 
-    it "calls code within `on_system :linux, macos: :big_sur_or_older` on Big Sur" do
+    it "calls code within `on_system :linux, macos: :big_sur_or_older` on Big Sur", :needs_macos do
       Homebrew::SimulateSystem.os = :big_sur
       f.brew { f.install }
       expect(f.foo).to eq(0)
       expect(f.bar).to eq(1)
     end
 
-    it "calls code within `on_system :linux, macos: :big_sur_or_older` on Catalina" do
+    it "calls code within `on_system :linux, macos: :big_sur_or_older` on Catalina", :needs_macos do
       Homebrew::SimulateSystem.os = :catalina
       f.brew { f.install }
       expect(f.foo).to eq(0)