diff --git a/js/src/ion/Ion.cpp b/js/src/ion/Ion.cpp
index c334ebf49d2e..60d629abd2f6 100644
--- a/js/src/ion/Ion.cpp
+++ b/js/src/ion/Ion.cpp
@@ -1287,7 +1287,7 @@ ion::CanEnter(JSContext *cx, HandleScript script, StackFrame *fp, bool newType)
 }
 
 MethodStatus
-ion::CanEnterUsingFastInvoke(JSContext *cx, HandleScript script)
+ion::CanEnterUsingFastInvoke(JSContext *cx, HandleScript script, uint32_t numActualArgs)
 {
     JS_ASSERT(ion::IsEnabled(cx));
 
@@ -1295,6 +1295,11 @@ ion::CanEnterUsingFastInvoke(JSContext *cx, HandleScript script)
     if (!script->hasIonScript() || script->ion->bailoutExpected())
         return Method_Skipped;
 
+    // Don't handle arguments underflow, to make this work we would have to pad
+    // missing arguments with |undefined|.
+    if (numActualArgs < script->function()->nargs)
+        return Method_Skipped;
+
     if (!cx->compartment->ensureIonCompartmentExists(cx))
         return Method_Error;
 
@@ -1491,11 +1496,12 @@ ion::FastInvoke(JSContext *cx, HandleFunction fun, CallArgsList &args)
     EnterIonCode enter = cx->compartment->ionCompartment()->enterJITInfallible();
     void *calleeToken = CalleeToToken(fun);
 
-    Value result = Int32Value(fun->nargs);
+    Value result = Int32Value(args.length());
+    JS_ASSERT(args.length() >= fun->nargs);
 
     JSAutoResolveFlags rf(cx, RESOLVE_INFER);
     args.setActive();
-    enter(jitcode, args.length() + 1, &args[0] - 1, fp, calleeToken, &result);
+    enter(jitcode, args.length() + 1, args.array() - 1, fp, calleeToken, &result);
     args.setInactive();
 
     if (clearCallingIntoIon)
diff --git a/js/src/ion/Ion.h b/js/src/ion/Ion.h
index 0f4de04f3d23..5d3e76475e16 100644
--- a/js/src/ion/Ion.h
+++ b/js/src/ion/Ion.h
@@ -226,7 +226,7 @@ bool SetIonContext(IonContext *ctx);
 MethodStatus CanEnterAtBranch(JSContext *cx, HandleScript script,
                               StackFrame *fp, jsbytecode *pc);
 MethodStatus CanEnter(JSContext *cx, HandleScript script, StackFrame *fp, bool newType);
-MethodStatus CanEnterUsingFastInvoke(JSContext *cx, HandleScript script);
+MethodStatus CanEnterUsingFastInvoke(JSContext *cx, HandleScript script, uint32_t numActualArgs);
 
 enum IonExecStatus
 {
diff --git a/js/src/jit-test/tests/ion/bug809021.js b/js/src/jit-test/tests/ion/bug809021.js
new file mode 100644
index 000000000000..c4c3d436be74
--- /dev/null
+++ b/js/src/jit-test/tests/ion/bug809021.js
@@ -0,0 +1,73 @@
+// nactuals > nformals
+function testOverflow() {
+    var called = false;
+    function f(a) {
+        assertEq(a, 173);
+        assertEq(arguments.length, 2);
+        assertEq(arguments[0], a);
+        assertEq(arguments[1], a);
+        called = true;
+    }
+
+    for (var i=0; i<10; i++)
+        [173, 173, 173].sort(f);
+    assertEq(called, true);
+}
+testOverflow();
+
+// nactuals == nformals
+function testEqual() {
+    var called = false;
+    function f(a, b) {
+        assertEq(a, 173);
+        assertEq(arguments.length, 2);
+        assertEq(arguments[0], a);
+        assertEq(arguments[1], b);
+        called = true;
+    }
+
+    for (var i=0; i<10; i++)
+        [173, 173, 173].sort(f);
+    assertEq(called, true);
+}
+testEqual();
+
+// nactuals < nformals
+function testUnderflow() {
+    var called = false;
+    function f(a, b, c) {
+        assertEq(a, 173);
+        assertEq(c, undefined);
+        assertEq(arguments.length, 2);
+        assertEq(arguments[0], a);
+        assertEq(arguments[1], b);
+        called = true;
+    }
+
+    for (var i=0; i<10; i++)
+        [173, 173, 173].sort(f);
+    assertEq(called, true);
+}
+testUnderflow();
+
+function testUnderflowMany() {
+    var called = 0;
+    function f(a, b, c, d, e, f, g, h) {
+        assertEq(a, 173);
+        assertEq(arguments.length, 3);
+        assertEq(arguments[0], a);
+        assertEq(arguments[1] < 3, true);
+        assertEq(c.length, 3);
+        assertEq(d, undefined);
+        assertEq(e, undefined);
+        assertEq(f, undefined);
+        assertEq(g, undefined);
+        assertEq(h, undefined);
+        called += 1;
+    }
+
+    for (var i=0; i<10; i++)
+        [173, 173, 173].map(f);
+    assertEq(called, 30);
+}
+testUnderflowMany();
diff --git a/js/src/jsinterpinlines.h b/js/src/jsinterpinlines.h
index 34f85d3f5373..9ae71c67e756 100644
--- a/js/src/jsinterpinlines.h
+++ b/js/src/jsinterpinlines.h
@@ -1031,7 +1031,7 @@ class FastInvokeGuard
         if (useIon_ && fun_) {
             JS_ASSERT(fun_->script() == script_);
 
-            ion::MethodStatus status = ion::CanEnterUsingFastInvoke(cx, script_);
+            ion::MethodStatus status = ion::CanEnterUsingFastInvoke(cx, script_, args_.length());
             if (status == ion::Method_Error)
                 return false;
             if (status == ion::Method_Compiled) {