diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index 664b926d0..335319c01 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -3598,6 +3598,20 @@ BytecodeEmitter::emitFunctionScript(ParseNode* body)
         switchToMain();
     }
 
+    if (funbox->isAsync()) {
+        // Currently short-circuit async functions with a throw.
+        // TenFourFox issue 521.
+        if (!emit1(JSOP_NULL))
+            return false;
+        if (!emit1(JSOP_THROW))
+            return false;
+        if (!emit1(JSOP_NULL))
+            return false;
+        if (!emit1(JSOP_RETURN))
+            return false;
+        goto asyncout;
+    }
+
     if (!emitTree(body))
         return false;
 
@@ -3648,6 +3662,7 @@ BytecodeEmitter::emitFunctionScript(ParseNode* body)
         if (!emit1(JSOP_CHECKRETURN))
             return false;
     }
+asyncout:
 
     // Always end the script with a JSOP_RETRVAL. Some other parts of the codebase
     // depend on this opcode, e.g. InterpreterRegs::setToEndOfScript.
diff --git a/js/src/tests/ecma_7/AsyncFunctions/104_basic.js b/js/src/tests/ecma_7/AsyncFunctions/104_basic.js
index 9a06599b0..97733ee7f 100644
--- a/js/src/tests/ecma_7/AsyncFunctions/104_basic.js
+++ b/js/src/tests/ecma_7/AsyncFunctions/104_basic.js
@@ -27,11 +27,12 @@ function okok(x, y, z) { return (typeof y === "function"); }
 assertEq(okok(5, async function(w) { await w+w; }, "ok"), true);
 assertEq(okok(6, (async(w)=>{await w+w}), "ok"), true);
 assertEq(okok(7, ()=>{!async function(){ }}, "ok"), true);
+assertEq(okok(8, async event => { }, "ok"), true);
 
 function yoyo(k) { return new Promise(resolve => { resolve(k+1); }); }
 async function dodo(k) { return await yoyo(k+1); }
-// Just make sure this executes. It currently returns ({ }) in the shell.
-dodo(5);
+// Just make sure this executes. Right now this throws.
+try { dodo(5); } catch(e) { }
 
 if (typeof reportCompare === "function")
     reportCompare(true, true);