diff --git a/lib/onelogin/ruby-saml/authrequest.rb b/lib/onelogin/ruby-saml/authrequest.rb
index 0aadff2f..732bcdda 100644
--- a/lib/onelogin/ruby-saml/authrequest.rb
+++ b/lib/onelogin/ruby-saml/authrequest.rb
@@ -64,7 +64,7 @@ def create_params(settings, params={})
         request_doc = create_authentication_xml_doc(settings)
         request_doc.context[:attribute_quote] = :quote if settings.double_quote_xml_attribute_values
 
-        request = ""
+        request = +""
         request_doc.write(request)
 
         Logging.debug "Created AuthnRequest: #{request}"
diff --git a/lib/onelogin/ruby-saml/logoutrequest.rb b/lib/onelogin/ruby-saml/logoutrequest.rb
index d2bb21ec..2508f17a 100644
--- a/lib/onelogin/ruby-saml/logoutrequest.rb
+++ b/lib/onelogin/ruby-saml/logoutrequest.rb
@@ -61,7 +61,7 @@ def create_params(settings, params={})
         request_doc = create_logout_request_xml_doc(settings)
         request_doc.context[:attribute_quote] = :quote if settings.double_quote_xml_attribute_values
 
-        request = ""
+        request = +""
         request_doc.write(request)
 
         Logging.debug "Created SLO Logout Request: #{request}"
diff --git a/lib/onelogin/ruby-saml/metadata.rb b/lib/onelogin/ruby-saml/metadata.rb
index fed96b67..d0f7cba3 100644
--- a/lib/onelogin/ruby-saml/metadata.rb
+++ b/lib/onelogin/ruby-saml/metadata.rb
@@ -145,7 +145,7 @@ def embed_signature(meta_doc, settings)
       end
 
       def output_xml(meta_doc, pretty_print)
-        ret = ''
+        ret = +''
 
         # pretty print the XML so IdP administrators can easily see what the SP supports
         if pretty_print
diff --git a/lib/onelogin/ruby-saml/slo_logoutresponse.rb b/lib/onelogin/ruby-saml/slo_logoutresponse.rb
index c2c73d0c..df36d537 100644
--- a/lib/onelogin/ruby-saml/slo_logoutresponse.rb
+++ b/lib/onelogin/ruby-saml/slo_logoutresponse.rb
@@ -70,7 +70,7 @@ def create_params(settings, request_id = nil, logout_message = nil, params = {},
         response_doc = create_logout_response_xml_doc(settings, request_id, logout_message, logout_status_code)
         response_doc.context[:attribute_quote] = :quote if settings.double_quote_xml_attribute_values
 
-        response = ""
+        response = +""
         response_doc.write(response)
 
         Logging.debug "Created SLO Logout Response: #{response}"
diff --git a/lib/onelogin/ruby-saml/utils.rb b/lib/onelogin/ruby-saml/utils.rb
index 68ee2ed0..ca5ac748 100644
--- a/lib/onelogin/ruby-saml/utils.rb
+++ b/lib/onelogin/ruby-saml/utils.rb
@@ -32,7 +32,7 @@ class Utils
           (\d+)W                    # 8: Weeks
         )
       $)x.freeze
-      UUID_PREFIX = '_'
+      UUID_PREFIX = +'_'
 
       # Checks if the x509 cert provided is expired.
       #
@@ -262,11 +262,11 @@ def self.status_error_msg(error_msg, raw_status_code = nil, status_message = nil
           else
             printable_code = raw_status_code.split(':').last
           end
-          error_msg << ', was ' + printable_code
+          error_msg += ', was ' + printable_code
         end
 
         unless status_message.nil?
-          error_msg << ' -> ' + status_message
+          error_msg += ' -> ' + status_message
         end
 
         error_msg