diff --git a/js/src/frontend/Parser.cpp b/js/src/frontend/Parser.cpp
index d92c1fdad611..b7da091baf2b 100644
--- a/js/src/frontend/Parser.cpp
+++ b/js/src/frontend/Parser.cpp
@@ -782,7 +782,7 @@ Define(ParseNode *pn, JSAtom *atom, TreeContext *tc, bool let = false)
             pnup = &pnu->pn_link;
         }
 
-        if (pnu != dn->dn_uses) {
+        if (!pnu || pnu != dn->dn_uses) {
             *pnup = pn->dn_uses;
             pn->dn_uses = dn->dn_uses;
             dn->dn_uses = pnu;
diff --git a/js/src/jit-test/tests/basic/bug778268.js b/js/src/jit-test/tests/basic/bug778268.js
new file mode 100644
index 000000000000..18cf41883507
--- /dev/null
+++ b/js/src/jit-test/tests/basic/bug778268.js
@@ -0,0 +1,3 @@
+// Just don't assert
+j : 4;
+let [j] = 1, j;