diff --git a/layout/style/StyleRule.cpp b/layout/style/StyleRule.cpp
index f4a3d05e5b..0ae939098a 100644
--- a/layout/style/StyleRule.cpp
+++ b/layout/style/StyleRule.cpp
@@ -1048,6 +1048,10 @@ nsCSSSelectorList::ToString(nsAString& aResult, CSSStyleSheet* aSheet)
 {
   aResult.Truncate();
   nsCSSSelectorList *p = this;
+  // Don't append anything if we're an empty selector list.
+  if (!mSelectors) {
+    return;
+  }
   for (;;) {
     p->mSelectors->ToString(aResult, aSheet, true);
     p = p->mNext;