diff --git a/lib/src/widgets/toolbar/buttons/dropdown_header_style.dart b/lib/src/widgets/toolbar/buttons/dropdown_header_style.dart
index f6ef06cb..36600716 100644
--- a/lib/src/widgets/toolbar/buttons/dropdown_header_style.dart
+++ b/lib/src/widgets/toolbar/buttons/dropdown_header_style.dart
@@ -251,7 +251,7 @@ class _QuillToolbarSelectHeaderStyleDropdownButtonState
               header.value,
               style: TextStyle(
                 fontSize: options.renderItemTextStyle
-                    ? _headerTextStyles![header.key]!.fontSize ??
+                    ? _headerStyle(header.key).fontSize ??
                         DefaultTextStyle.of(context).style.fontSize ??
                         14
                     : null,
@@ -275,4 +275,9 @@ class _QuillToolbarSelectHeaderStyleDropdownButtonState
     controller.formatSelection(attribute0);
     afterButtonPressed?.call();
   }
+
+  TextStyle _headerStyle(Attribute attribute) {
+    assert(_headerTextStyles!.containsKey(attribute));
+    return _headerTextStyles![attribute]!;
+  }
 }