diff --git a/lib/axlsx/workbook/worksheet/cell.rb b/lib/axlsx/workbook/worksheet/cell.rb
index <HASH>..<HASH> 100644
--- a/lib/axlsx/workbook/worksheet/cell.rb
+++ b/lib/axlsx/workbook/worksheet/cell.rb
@@ -67,7 +67,7 @@ module Axlsx
     # Indicates if the cell is good for shared string table
     def plain_string?
       @type == :string &&         # String typed
-        !@is_text_run &&          # No inline styles
+        !is_text_run? &&          # No inline styles
         !@value.nil? &&           # Not nil
         !@value.empty? &&         # Not empty
         !@value.start_with?('=')  # Not a formula
use method not instance variable for determining if cell level style customizations have been made and the cell is now a text run.