diff --git a/Library/Homebrew/cmd/readall.rb b/Library/Homebrew/cmd/readall.rb
index 9216f63770..260a1750d1 100644
--- a/Library/Homebrew/cmd/readall.rb
+++ b/Library/Homebrew/cmd/readall.rb
@@ -35,7 +35,7 @@ module Homebrew
       formulae = Formula.full_names
     else
       tap = Tap.new(*tap_args)
-      raise "#{tap} does not exist!" unless tap.installed?
+      raise TapUnavailableError, tap.name unless tap.installed?
       formulae = tap.formula_files
     end
 
diff --git a/Library/Homebrew/cmd/untap.rb b/Library/Homebrew/cmd/untap.rb
index dd66cec1be..f1040bb051 100644
--- a/Library/Homebrew/cmd/untap.rb
+++ b/Library/Homebrew/cmd/untap.rb
@@ -7,7 +7,7 @@ module Homebrew
     ARGV.named.each do |tapname|
       tap = Tap.new(*tap_args(tapname))
 
-      raise "No such tap!" unless tap.installed?
+      raise TapUnavailableError, tap.name unless tap.installed?
       puts "Untapping #{tap}... (#{tap.path.abv})"
 
       formula_count = tap.formula_files.size
diff --git a/Library/Homebrew/exceptions.rb b/Library/Homebrew/exceptions.rb
index 7ae185d00d..7eb053a7c9 100644
--- a/Library/Homebrew/exceptions.rb
+++ b/Library/Homebrew/exceptions.rb
@@ -84,6 +84,18 @@ class TapFormulaAmbiguityError < RuntimeError
   end
 end
 
+class TapUnavailableError < RuntimeError
+  attr_reader :name
+
+  def initialize name
+    @name = name
+
+    super <<-EOS.undent
+      No available tap #{name}.
+    EOS
+  end
+end
+
 class OperationInProgressError < RuntimeError
   def initialize name
     message = <<-EOS.undent