diff --git a/content/base/src/nsDocument.cpp b/content/base/src/nsDocument.cpp
index 4071426dc1f..0b7cafc95e0 100644
--- a/content/base/src/nsDocument.cpp
+++ b/content/base/src/nsDocument.cpp
@@ -2625,6 +2625,9 @@ nsDocument::RemoveFromNameTable(Element *aElement, nsIAtom* aName)
 void
 nsDocument::AddToIdTable(Element *aElement, nsIAtom* aId)
 {
+  NS_ASSERTION(!nsContentUtils::IsSafeToRunScript(),
+               "Scripts should be blocked when manipulating the ID table");
+
   nsIdentifierMapEntry *entry =
     mIdentifierMap.PutEntry(nsDependentAtomString(aId));
 
@@ -2637,6 +2640,8 @@ void
 nsDocument::RemoveFromIdTable(Element *aElement, nsIAtom* aId)
 {
   NS_ASSERTION(aId, "huhwhatnow?");
+  NS_ASSERTION(!nsContentUtils::IsSafeToRunScript(),
+               "Scripts should be blocked when manipulating the ID table");
 
   // Speed up document teardown
   if (mIdentifierMap.Count() == 0) {