diff --git a/image/SVGDocumentWrapper.cpp b/image/SVGDocumentWrapper.cpp
index 1684c602ecdc..08228c9c708c 100644
--- a/image/SVGDocumentWrapper.cpp
+++ b/image/SVGDocumentWrapper.cpp
@@ -54,6 +54,7 @@ SVGDocumentWrapper::~SVGDocumentWrapper() {
 }
 
 void SVGDocumentWrapper::DestroyViewer() {
+  MOZ_ASSERT(NS_IsMainThread());
   if (mViewer) {
     mViewer->GetDocument()->OnPageHide(false, nullptr);
     mViewer->Close(nullptr);