diff --git a/gfx/webrender_bindings/webrender_ffi.h b/gfx/webrender_bindings/webrender_ffi.h
index d04721a4aacb..e57dd636ea15 100644
--- a/gfx/webrender_bindings/webrender_ffi.h
+++ b/gfx/webrender_bindings/webrender_ffi.h
@@ -69,7 +69,7 @@ void* get_proc_address_from_glcontext(void* glcontext_ptr, const char* procname)
 // -----
 // Enums used in C++ code with corresponding enums in Rust code
 // -----
-enum class WrBoxShadowClipMode {
+enum class WrBoxShadowClipMode: uint32_t {
   None,
   Outset,
   Inset,