diff --git a/src/request_tab_widget.py b/src/request_tab_widget.py
index cd03873..dc33794 100644
--- a/src/request_tab_widget.py
+++ b/src/request_tab_widget.py
@@ -735,6 +735,7 @@ class RequestTabWidget(Gtk.Box):
         """Update visual indicators for undefined variables."""
         # Detect undefined variables
         self.undefined_variables = self._detect_undefined_variables()
+        print(f"Updating indicators, undefined vars: {self.undefined_variables}")
 
         # Update URL entry
         self._update_url_indicator()
@@ -769,33 +770,45 @@ class RequestTabWidget(Gtk.Box):
         # Iterate through all header rows
         child = self.headers_listbox.get_first_child()
         while child:
-            # Check if this is a HeaderRow widget
+            # Debug: print child type
+            print(f"Header child type: {type(child).__name__}")
+
+            # HeaderRow might be a child of the ListBox row
+            header_row = None
             if isinstance(child, HeaderRow):
+                header_row = child
+            elif hasattr(child, 'get_child'):
+                # GTK4 ListBox wraps widgets in GtkListBoxRow
+                actual_child = child.get_child()
+                if isinstance(actual_child, HeaderRow):
+                    header_row = actual_child
+
+            if header_row:
                 # Check key
-                key_text = child.key_entry.get_text()
+                key_text = header_row.key_entry.get_text()
                 key_vars = set(VariableSubstitution.find_variables(key_text))
                 undefined_in_key = key_vars & self.undefined_variables
 
                 if undefined_in_key:
-                    child.key_entry.add_css_class("warning")
+                    header_row.key_entry.add_css_class("warning")
                     tooltip = "Undefined: " + ", ".join(sorted(undefined_in_key))
-                    child.key_entry.set_tooltip_text(tooltip)
+                    header_row.key_entry.set_tooltip_text(tooltip)
                 else:
-                    child.key_entry.remove_css_class("warning")
-                    child.key_entry.set_tooltip_text("")
+                    header_row.key_entry.remove_css_class("warning")
+                    header_row.key_entry.set_tooltip_text("")
 
                 # Check value
-                value_text = child.value_entry.get_text()
+                value_text = header_row.value_entry.get_text()
                 value_vars = set(VariableSubstitution.find_variables(value_text))
                 undefined_in_value = value_vars & self.undefined_variables
 
                 if undefined_in_value:
-                    child.value_entry.add_css_class("warning")
+                    header_row.value_entry.add_css_class("warning")
                     tooltip = "Undefined: " + ", ".join(sorted(undefined_in_value))
-                    child.value_entry.set_tooltip_text(tooltip)
+                    header_row.value_entry.set_tooltip_text(tooltip)
                 else:
-                    child.value_entry.remove_css_class("warning")
-                    child.value_entry.set_tooltip_text("")
+                    header_row.value_entry.remove_css_class("warning")
+                    header_row.value_entry.set_tooltip_text("")
 
             child = child.get_next_sibling()