diff --git a/lib/rails_xss/string_ext.rb b/lib/rails_xss/string_ext.rb
index ae21705..ed1aaa2 100644
--- a/lib/rails_xss/string_ext.rb
+++ b/lib/rails_xss/string_ext.rb
@@ -9,6 +9,19 @@ ActiveSupport::SafeBuffer.class_eval do
     end
   end
   alias << concat
+  UNSAFE_STRING_METHODS = ["capitalize", "chomp", "chop", "delete", "downcase", "gsub", "lstrip", "next", "reverse", "rstrip", "slice", "squeeze", "strip", "sub", "succ", "swapcase", "tr", "tr_s", "upcase"].freeze
+
+  for unsafe_method in UNSAFE_STRING_METHODS
+    class_eval <<-EOT, __FILE__, __LINE__
+      def #{unsafe_method}(*args)
+        super.to_str
+      end
+  
+      def #{unsafe_method}!(*args)
+        raise TypeError, "Cannot modify SafeBuffer in place"
+      end
+    EOT
+  end
 end
 
 class String
diff --git a/test/safe_buffer_test.rb b/test/safe_buffer_test.rb
index 6a515da..cf3b5f5 100644
--- a/test/safe_buffer_test.rb
+++ b/test/safe_buffer_test.rb
@@ -38,4 +38,14 @@ class SafeBufferTest < ActiveSupport::TestCase
     new_buffer = @buffer.to_s
     assert_equal ActiveSupport::SafeBuffer, new_buffer.class
   end
+  
+  test "Should not return a safe buffer when using sub" do
+    assert !@buffer.sub('', "asdf").html_safe?
+  end
+  
+  test "Should raise argument error when using sub!" do
+    assert_raise ArgumentError do
+      @buffer.sub!('', "asdf")
+    end
+  end
 end