diff --git a/lib/creek/book.rb b/lib/creek/book.rb
index fae6090..1bb26e5 100644
--- a/lib/creek/book.rb
+++ b/lib/creek/book.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'zip/filesystem'
 require 'nokogiri'
 require 'date'
diff --git a/lib/creek/drawing.rb b/lib/creek/drawing.rb
index efba81a..7847536 100644
--- a/lib/creek/drawing.rb
+++ b/lib/creek/drawing.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'pathname'
 
 module Creek
diff --git a/lib/creek/shared_strings.rb b/lib/creek/shared_strings.rb
index 1825101..663aaf1 100644
--- a/lib/creek/shared_strings.rb
+++ b/lib/creek/shared_strings.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'zip/filesystem'
 require 'nokogiri'
 
diff --git a/lib/creek/sheet.rb b/lib/creek/sheet.rb
index be30c51..1df94b8 100644
--- a/lib/creek/sheet.rb
+++ b/lib/creek/sheet.rb
@@ -99,6 +99,10 @@ def rows_generator include_meta_data=false, use_simple_rows_format=false
           cell_style_idx = nil
           @book.files.file.open(path) do |xml|
             prefix = ''
+            name_row = "row"
+            name_c = "c"
+            name_v = "v"
+            name_t = "t"
             Nokogiri::XML::Reader.from_io(xml).each do |node|
               if prefix.empty? && node.namespaces.any?
                 namespace = node.namespaces.detect{|_key, uri| uri == SPREADSHEETML_URI }
@@ -107,13 +111,17 @@ def rows_generator include_meta_data=false, use_simple_rows_format=false
                          else
                            ''
                          end
+                name_row = "#{prefix}row"
+                name_c = "#{prefix}c"
+                name_v = "#{prefix}v"
+                name_t = "#{prefix}t"
               end
-              if node.name == "#{prefix}row" && node.node_type == opener
+              if node.name == name_row && node.node_type == opener
                 row = node.attributes
                 row['cells'] = {}
                 cells = {}
                 y << (include_meta_data ? row : cells) if node.self_closing?
-              elsif node.name == "#{prefix}row" && node.node_type == closer
+              elsif node.name == name_row && node.node_type == closer
                 processed_cells = fill_in_empty_cells(cells, row['r'], cell, use_simple_rows_format)
                 @headers = processed_cells if with_headers && row['r'] == HEADERS_ROW_NUMBER
 
@@ -127,11 +135,11 @@ def rows_generator include_meta_data=false, use_simple_rows_format=false
 
                 row['cells'] = processed_cells
                 y << (include_meta_data ? row : processed_cells)
-              elsif node.name == "#{prefix}c" && node.node_type == opener
+              elsif node.name == name_c && node.node_type == opener
                 cell_type      = node.attributes['t']
                 cell_style_idx = node.attributes['s']
                 cell           = node.attributes['r']
-              elsif ["#{prefix}v", "#{prefix}t"].include?(node.name) && node.node_type == opener
+              elsif (node.name == name_v || node.name == name_t) && node.node_type == opener
                 unless cell.nil?
                   node.read
                   cells[cell] = convert(node.value, cell_type, cell_style_idx)
diff --git a/lib/creek/styles.rb b/lib/creek/styles.rb
index d4681e9..016f158 100644
--- a/lib/creek/styles.rb
+++ b/lib/creek/styles.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Creek
   class Styles
     attr_accessor :book
diff --git a/lib/creek/styles/converter.rb b/lib/creek/styles/converter.rb
index afe1eb6..62bd6d6 100644
--- a/lib/creek/styles/converter.rb
+++ b/lib/creek/styles/converter.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'set'
 
 module Creek
diff --git a/lib/creek/styles/style_types.rb b/lib/creek/styles/style_types.rb
index de0db03..df3b4d1 100644
--- a/lib/creek/styles/style_types.rb
+++ b/lib/creek/styles/style_types.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # https://github.com/hmcgowan/roo/blob/master/lib/roo/excelx.rb
 # https://github.com/woahdae/simple_xlsx_reader/blob/master/lib/simple_xlsx_reader.rb#L231
 module Creek
diff --git a/lib/creek/utils.rb b/lib/creek/utils.rb
index 9eef47c..af4ac90 100644
--- a/lib/creek/utils.rb
+++ b/lib/creek/utils.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Creek
   module Utils
     def expand_to_rels_path(filepath)
diff --git a/lib/creek/version.rb b/lib/creek/version.rb
index 1d6ad28..2073d94 100644
--- a/lib/creek/version.rb
+++ b/lib/creek/version.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Creek
   VERSION = "2.6.2"
 end