diff --git a/src/book.py b/src/book.py
index f3d95a3..e340ad6 100644
--- a/src/book.py
+++ b/src/book.py
@@ -851,12 +851,15 @@ class Book:
             self.set_error_status()
             raise exc
 
-        # add is_introduction field if first chapter is Introduction
-        if not self.top_level_headers[0]['should_be_numbered']:
-            # title should be checked as after deleting content
-            # before toc, some chapters can be deleted
-            if self.top_level_headers[0]['title'] in json_strc[0].keys():
-                json_strc[0]['is_introduction'] = True
+        # Add is_introduction field to json structure
+        # after deleting content before toc, some chapters can be deleted
+        same_first_titles = self.top_level_headers[0]['title'] in json_strc[0].keys()
+        is_first_header_introduction = not self.top_level_headers[0]['should_be_numbered']
+
+        if is_first_header_introduction and same_first_titles:
+            json_strc[0]['is_introduction'] = True
+        else:
+            json_strc[0]['is_introduction'] = False
 
         self.content_dict = {
             "content": json_strc,