diff --git a/gfx/src/X11Util.cpp b/gfx/src/X11Util.cpp
index ac9c25958248..8cdf145e45be 100644
--- a/gfx/src/X11Util.cpp
+++ b/gfx/src/X11Util.cpp
@@ -7,6 +7,7 @@
 
 #include "X11Util.h"
 #include "nsDebug.h"                    // for NS_ASSERTION, etc
+#include "MainThreadUtils.h"            // for NS_IsMainThread
 
 namespace mozilla {
 
@@ -58,6 +59,7 @@ ScopedXErrorHandler::ErrorHandler(Display *, XErrorEvent *ev)
 
 ScopedXErrorHandler::ScopedXErrorHandler()
 {
+    MOZ_ASSERT(NS_IsMainThread());
     // let sXErrorPtr point to this object's mXError object, but don't reset this mXError object!
     // think of the case of nested ScopedXErrorHandler's.
     mOldXErrorPtr = sXErrorPtr;