diff --git a/ui/accessibility/platform/ax_platform_node_cocoa.mm b/ui/accessibility/platform/ax_platform_node_cocoa.mm
index 0a7e17535525e..7fd38b9fbee05 100644
--- a/ui/accessibility/platform/ax_platform_node_cocoa.mm
+++ b/ui/accessibility/platform/ax_platform_node_cocoa.mm
@@ -2219,6 +2219,11 @@ const ui::CocoaActionList& GetCocoaActionListForTesting() {
     case ax::mojom::InvalidState::kTrue:
       return @"true";
   }
+
+  // The underlying int_attribute is renderer-supplied and not range-checked
+  // by Mojo deserialisation; an out-of-range value must not be allowed to
+  // fall off the end of this non-void method.
+  NOTREACHED();
 }
 
 - (NSNumber*)AXIsMultiSelectable {