diff --git a/Library/Homebrew/cmd/switch.rb b/Library/Homebrew/cmd/switch.rb
index a40b4d88e7..8a378aed79 100644
--- a/Library/Homebrew/cmd/switch.rb
+++ b/Library/Homebrew/cmd/switch.rb
@@ -28,7 +28,7 @@ module Homebrew
     unless (rack/version).directory?
       onoe "#{name} does not have a version \"#{version}\" in the Cellar."
 
-      versions = rack.subdirs.map { |d| Keg.new(d).version }
+      versions = rack.subdirs.map { |d| Keg.new(d).version }.sort
       puts "Versions available: #{versions.join(", ")}"
 
       exit 3
diff --git a/Library/Homebrew/formula.rb b/Library/Homebrew/formula.rb
index d64191b66b..66993a348b 100644
--- a/Library/Homebrew/formula.rb
+++ b/Library/Homebrew/formula.rb
@@ -602,7 +602,7 @@ class Formula
   # All currently installed prefix directories.
   # @private
   def installed_prefixes
-    rack.directory? ? rack.subdirs : []
+    rack.directory? ? rack.subdirs.sort : []
   end
 
   # All currently installed kegs.
@@ -1182,7 +1182,7 @@ class Formula
     return false unless old_rack.directory?
     return false if old_rack.subdirs.empty?
 
-    tap == Tab.for_keg(old_rack.subdirs.first).tap
+    tap == Tab.for_keg(old_rack.subdirs.sort.first).tap
   end
 
   # @private
diff --git a/Library/Homebrew/test/formula_pin_spec.rb b/Library/Homebrew/test/formula_pin_spec.rb
index 909bfbc2b3..ce952a9574 100644
--- a/Library/Homebrew/test/formula_pin_spec.rb
+++ b/Library/Homebrew/test/formula_pin_spec.rb
@@ -9,7 +9,7 @@ describe FormulaPin do
     formula.rack.mkpath
 
     allow(formula).to receive(:installed_prefixes) do
-      formula.rack.directory? ? formula.rack.subdirs : []
+      formula.rack.directory? ? formula.rack.subdirs.sort : []
     end
 
     allow(formula).to receive(:installed_kegs) do