diff --git a/lib/pry/pry_instance.rb b/lib/pry/pry_instance.rb
index 10c28124..6e99f231 100644
--- a/lib/pry/pry_instance.rb
+++ b/lib/pry/pry_instance.rb
@@ -533,7 +533,8 @@ class Pry
     #   valid_expression?("class Hello") #=> false
     #   valid_expression?("class Hello; end") #=> true
     def valid_expression?(code)
-      RubyParser.new.parse(code)
+      # NOTE: we're using .dup because RubyParser mutates the input
+      RubyParser.new.parse(code.dup)
       true
     rescue Racc::ParseError, SyntaxError
       false
diff --git a/test/test_pry.rb b/test/test_pry.rb
index 636d32d0..5023a5be 100644
--- a/test/test_pry.rb
+++ b/test/test_pry.rb
@@ -207,6 +207,15 @@ describe Pry do
         end
       end
 
+      describe "valid_expression?" do
+        it "should not mutate the input!" do
+          clean = "puts <<-FOO\nhi\nFOO\n"
+          a = clean.dup
+          Pry.new.valid_expression?(a)
+          a.should == clean
+        end
+      end
+
       describe "history arrays" do
         it 'sets _ to the last result' do
           res   = []