--- a/layout/style/StyleSheet.cpp
+++ b/layout/style/StyleSheet.cpp
@@ -1187,17 +1187,22 @@ uint32_t StyleSheet::InsertRuleInternal(
 }
 
 void StyleSheet::DeleteRuleInternal(uint32_t aIndex, ErrorResult& aRv) {
   MOZ_ASSERT(!IsReadOnly());
 
   // Ensure mRuleList is constructed.
   GetCssRulesInternal();
   if (aIndex >= mRuleList->Length()) {
-    aRv.Throw(NS_ERROR_DOM_INDEX_SIZE_ERR);
+    aRv.ThrowDOMException(
+        NS_ERROR_DOM_INDEX_SIZE_ERR,
+        nsPrintfCString(
+            "Cannot delete rule at index %u"
+            " because the number of rules is only %u",
+            aIndex, mRuleList->Length()));
     return;
   }
 
   // Hold a strong ref to the rule so it doesn't die when we remove it
   // from the list. XXX We may not want to hold it if stylesheet change
   // event is not enabled.
   RefPtr<css::Rule> rule = mRuleList->GetRule(aIndex);
   aRv = mRuleList->DeleteRule(aIndex);