--- a/js/src/jsparse.cpp
+++ b/js/src/jsparse.cpp
@@ -6055,28 +6055,37 @@ BumpStaticLevel(JSParseNode *pn, JSTreeC
     if (pn->pn_cookie != FREE_UPVAR_COOKIE) {
         uintN level = UPVAR_FRAME_SKIP(pn->pn_cookie) + 1;
 
         JS_ASSERT(level >= tc->staticLevel);
         pn->pn_cookie = MAKE_UPVAR_COOKIE(level, UPVAR_FRAME_SLOT(pn->pn_cookie));
     }
 }
 
+static void
+AdjustBlockId(JSParseNode *pn, uintN adjust, JSTreeContext *tc)
+{
+    JS_ASSERT(pn->pn_arity == PN_LIST || pn->pn_arity == PN_FUNC || pn->pn_arity == PN_NAME);
+    pn->pn_blockid += adjust;
+    if (pn->pn_blockid >= tc->blockidGen)
+        tc->blockidGen = pn->pn_blockid + 1;
+}
+
 bool
 CompExprTransplanter::transplant(JSParseNode *pn)
 {
     if (!pn)
         return true;
 
     switch (pn->pn_arity) {
       case PN_LIST:
         for (JSParseNode *pn2 = pn->pn_head; pn2; pn2 = pn2->pn_next)
             transplant(pn2);
         if (pn->pn_pos >= root->pn_pos)
-            pn->pn_blockid += adjust;
+            AdjustBlockId(pn, adjust, tc);
         break;
 
       case PN_TERNARY:
         transplant(pn->pn_kid1);
         transplant(pn->pn_kid2);
         transplant(pn->pn_kid3);
         break;
 
@@ -6142,17 +6151,17 @@ CompExprTransplanter::transplant(JSParse
              * multiple times).
              *
              * Non-placeholder definitions within the comprehension expression
              * will be visited further below.
              */
             if (dn->isPlaceholder() && dn->pn_pos >= root->pn_pos && dn->dn_uses == pn) {
                 if (genexp)
                     BumpStaticLevel(dn, tc);
-                dn->pn_blockid += adjust;
+                AdjustBlockId(dn, adjust, tc);
             }
 
             JSAtom *atom = pn->pn_atom;
 #ifdef DEBUG
             JSStmtInfo *stmt = js_LexicalLookup(tc, atom, NULL);
             JS_ASSERT(!stmt || stmt != tc->topStmt);
 #endif
             if (genexp && PN_OP(dn) != JSOP_CALLEE) {
@@ -6191,17 +6200,17 @@ CompExprTransplanter::transplant(JSParse
                     }
 
                     ALE_SET_DEFN(ale, dn);
                 }
             }
         }
 
         if (pn->pn_pos >= root->pn_pos)
-            pn->pn_blockid += adjust;
+            AdjustBlockId(pn, adjust, tc);
         break;
 
       case PN_NAMESET:
         transplant(pn->pn_tree);
         break;
     }
     return true;
 }