diff --git a/js/src/jit-test/tests/ion/bug1092833.js b/js/src/jit-test/tests/ion/bug1092833.js
new file mode 100644
index 00000000000..64a74fb28fd
--- /dev/null
+++ b/js/src/jit-test/tests/ion/bug1092833.js
@@ -0,0 +1,49 @@
+// Test that lexicals work with functions with many bindings.
+
+(function() {
+    var a01
+    var b02
+    var c03
+    var d04
+    var e05
+    var f06
+    var g07
+    var h08
+    let i09
+    var j10
+    var k11
+    var l12
+    var m13
+    var n14
+    var o15
+    (function n14() {
+      assertEq(i09, undefined);
+    })()
+})();
+
+try {
+  (function() {
+      var a01
+      var b02
+      var c03
+      var d04
+      var e05
+      var f06
+      var g07
+      var h08
+      let i09
+      var j10
+      var k11
+      var l12
+      var m13
+      var n14
+      var o15
+      (function n14() {
+        i12++
+      })()
+      let i12
+  })()
+} catch (e) {
+  assertEq(e instanceof ReferenceError, true);
+  assertEq(e.message.indexOf("i12") > 0, true);
+}
diff --git a/js/src/jit/IonMacroAssembler.cpp b/js/src/jit/IonMacroAssembler.cpp
index 397bd044f7e..11b8701b38f 100644
--- a/js/src/jit/IonMacroAssembler.cpp
+++ b/js/src/jit/IonMacroAssembler.cpp
@@ -27,6 +27,7 @@
 #endif
 #include "jsinferinlines.h"
 #include "jsobjinlines.h"
+#include "vm/Interpreter-inl.h"
 
 using namespace js;
 using namespace js::jit;
@@ -956,12 +957,18 @@ MacroAssembler::copySlotsFromTemplate(Register obj, const NativeObject *template
 }
 
 void
-MacroAssembler::fillSlotsWithUndefined(Address base, Register temp, uint32_t start, uint32_t end)
+MacroAssembler::fillSlotsWithConstantValue(Address base, Register temp,
+                                           uint32_t start, uint32_t end, const Value &v)
 {
+    MOZ_ASSERT(v.isUndefined() || IsUninitializedLexical(v));
+
+    if (start >= end)
+        return;
+
 #ifdef JS_NUNBOX32
     // We only have a single spare register, so do the initialization as two
     // strided writes of the tag and body.
-    jsval_layout jv = JSVAL_TO_IMPL(UndefinedValue());
+    jsval_layout jv = JSVAL_TO_IMPL(v);
 
     Address addr = base;
     move32(Imm32(jv.s.payload.i32), temp);
@@ -973,22 +980,43 @@ MacroAssembler::fillSlotsWithUndefined(Address base, Register temp, uint32_t sta
     for (unsigned i = start; i < end; ++i, addr.offset += sizeof(HeapValue))
         store32(temp, ToType(addr));
 #else
-    moveValue(UndefinedValue(), temp);
+    moveValue(v, temp);
     for (uint32_t i = start; i < end; ++i, base.offset += sizeof(HeapValue))
         storePtr(temp, base);
 #endif
 }
 
-static uint32_t
-FindStartOfUndefinedSlots(NativeObject *templateObj, uint32_t nslots)
+void
+MacroAssembler::fillSlotsWithUndefined(Address base, Register temp, uint32_t start, uint32_t end)
+{
+    fillSlotsWithConstantValue(base, temp, start, end, UndefinedValue());
+}
+
+void
+MacroAssembler::fillSlotsWithUninitialized(Address base, Register temp, uint32_t start, uint32_t end)
+{
+    fillSlotsWithConstantValue(base, temp, start, end, MagicValue(JS_UNINITIALIZED_LEXICAL));
+}
+
+static void
+FindStartOfUndefinedAndUninitializedSlots(NativeObject *templateObj, uint32_t nslots,
+                                          uint32_t *startOfUndefined, uint32_t *startOfUninitialized)
 {
     MOZ_ASSERT(nslots == templateObj->lastProperty()->slotSpan(templateObj->getClass()));
     MOZ_ASSERT(nslots > 0);
-    for (uint32_t first = nslots; first != 0; --first) {
-        if (templateObj->getSlot(first - 1) != UndefinedValue())
-            return first;
+    uint32_t first = nslots;
+    for (; first != 0; --first) {
+        if (!IsUninitializedLexical(templateObj->getSlot(first - 1)))
+            break;
     }
-    return 0;
+    *startOfUninitialized = first;
+    for (; first != 0; --first) {
+        if (templateObj->getSlot(first - 1) != UndefinedValue()) {
+            *startOfUndefined = first;
+            return;
+        }
+    }
+    *startOfUndefined = 0;
 }
 
 void
@@ -1011,16 +1039,28 @@ MacroAssembler::initGCSlots(Register obj, Register slots, NativeObject *template
     // logically into independent non-UndefinedValue writes to the head and
     // duplicated writes of UndefinedValue to the tail. For the majority of
     // objects, the "tail" will be the entire slot range.
-    uint32_t startOfUndefined = FindStartOfUndefinedSlots(templateObj, nslots);
+    //
+    // The template object may be a CallObject, in which case we need to
+    // account for uninitialized lexical slots as well as undefined
+    // slots. Unitialized lexical slots always appear at the very end of
+    // slots, after undefined.
+    uint32_t startOfUndefined = nslots;
+    uint32_t startOfUninitialized = nslots;
+    FindStartOfUndefinedAndUninitializedSlots(templateObj, nslots,
+                                              &startOfUndefined, &startOfUninitialized);
     MOZ_ASSERT(startOfUndefined <= nfixed); // Reserved slots must be fixed.
+    MOZ_ASSERT_IF(startOfUndefined != nfixed, startOfUndefined <= startOfUninitialized);
+    MOZ_ASSERT_IF(!templateObj->is<CallObject>(), startOfUninitialized == nslots);
 
     // Copy over any preserved reserved slots.
     copySlotsFromTemplate(obj, templateObj, 0, startOfUndefined);
 
-    // Fill the rest of the fixed slots with undefined.
+    // Fill the rest of the fixed slots with undefined and uninitialized.
     if (initFixedSlots) {
         fillSlotsWithUndefined(Address(obj, NativeObject::getFixedSlotOffset(startOfUndefined)), slots,
-                               startOfUndefined, nfixed);
+                               startOfUndefined, Min(startOfUninitialized, nfixed));
+        size_t offset = NativeObject::getFixedSlotOffset(startOfUninitialized);
+        fillSlotsWithUninitialized(Address(obj, offset), slots, startOfUninitialized, nfixed);
     }
 
     if (ndynamic) {
@@ -1028,7 +1068,14 @@ MacroAssembler::initGCSlots(Register obj, Register slots, NativeObject *template
         // register briefly for our slots base address.
         push(obj);
         loadPtr(Address(obj, NativeObject::offsetOfSlots()), obj);
+
+        // Initially fill all dynamic slots with undefined.
         fillSlotsWithUndefined(Address(obj, 0), slots, 0, ndynamic);
+
+        // Fill uninitialized slots if necessary.
+        fillSlotsWithUninitialized(Address(obj, 0), slots, startOfUninitialized - nfixed,
+                                   nslots - startOfUninitialized);
+
         pop(obj);
     }
 }
diff --git a/js/src/jit/IonMacroAssembler.h b/js/src/jit/IonMacroAssembler.h
index a303a814fd9..7c8edc77a72 100644
--- a/js/src/jit/IonMacroAssembler.h
+++ b/js/src/jit/IonMacroAssembler.h
@@ -810,7 +810,10 @@ class MacroAssembler : public MacroAssemblerSpecific
     void allocateNonObject(Register result, Register temp, gc::AllocKind allocKind, Label *fail);
     void copySlotsFromTemplate(Register obj, const NativeObject *templateObj,
                                uint32_t start, uint32_t end);
+    void fillSlotsWithConstantValue(Address addr, Register temp, uint32_t start, uint32_t end,
+                                    const Value &v);
     void fillSlotsWithUndefined(Address addr, Register temp, uint32_t start, uint32_t end);
+    void fillSlotsWithUninitialized(Address addr, Register temp, uint32_t start, uint32_t end);
     void initGCSlots(Register obj, Register temp, NativeObject *templateObj, bool initFixedSlots);
 
   public: