diff --git a/widget/windows/nsAppShell.cpp b/widget/windows/nsAppShell.cpp
index c5fbebffa630..15d0d149c48e 100644
--- a/widget/windows/nsAppShell.cpp
+++ b/widget/windows/nsAppShell.cpp
@@ -534,7 +534,7 @@ nsresult nsAppShell::Init() {
       wc.hbrBackground = (HBRUSH) nullptr;
       wc.lpszMenuName = (LPCWSTR) nullptr;
       wc.lpszClassName = kWindowClass;
-      ATOM wcA = RegisterClassW(&wc);
+      [[maybe_unused]] ATOM wcA = RegisterClassW(&wc);
       MOZ_DIAGNOSTIC_ASSERT(wcA, "RegisterClassW for EventWindowClass failed");
     }