diff --git a/artiq/compiler/embedding.py b/artiq/compiler/embedding.py
index 806a0f856..97e3fb43d 100644
--- a/artiq/compiler/embedding.py
+++ b/artiq/compiler/embedding.py
@@ -139,11 +139,15 @@ class ASTSynthesizer:
                 return asttyped.QuoteT(value=value, type=instance_type,
                                        loc=loc)
 
-    def call(self, function_node, args, kwargs):
+    def call(self, function_node, args, kwargs, callback=None):
         """
         Construct an AST fragment calling a function specified by
         an AST node `function_node`, with given arguments.
         """
+        if callback is not None:
+            callback_node = self.quote(callback)
+            cb_begin_loc  = self._add("(")
+
         arg_nodes   = []
         kwarg_nodes = []
         kwarg_locs  = []
@@ -165,7 +169,10 @@ class ASTSynthesizer:
                          self._add(", ")
         end_loc        = self._add(")")
 
-        return asttyped.CallT(
+        if callback is not None:
+            cb_end_loc    = self._add(")")
+
+        node = asttyped.CallT(
             func=asttyped.NameT(id=function_node.name, ctx=None,
                                 type=function_node.signature_type,
                                 loc=name_loc),
@@ -180,6 +187,16 @@ class ASTSynthesizer:
             begin_loc=begin_loc, end_loc=end_loc, star_loc=None, dstar_loc=None,
             loc=name_loc.join(end_loc))
 
+        if callback is not None:
+            node = asttyped.CallT(
+                func=callback_node,
+                args=[node], keywords=[], starargs=None, kwargs=None,
+                type=builtins.TNone(), iodelay=None,
+                begin_loc=cb_begin_loc, end_loc=cb_end_loc, star_loc=None, dstar_loc=None,
+                loc=callback_node.loc.join(cb_end_loc))
+
+        return node
+
     def assign_local(self, var_name, value):
         name_loc   = self._add(var_name)
         _          = self._add(" ")
@@ -426,14 +443,14 @@ class Stitcher:
         self.type_map = {}
         self.value_map = defaultdict(lambda: [])
 
-    def stitch_call(self, function, args, kwargs):
+    def stitch_call(self, function, args, kwargs, callback=None):
         function_node = self._quote_embedded_function(function)
         self.typedtree.append(function_node)
 
         # We synthesize source code for the initial call so that
         # diagnostics would have something meaningful to display to the user.
         synthesizer = self._synthesizer()
-        call_node = synthesizer.call(function_node, args, kwargs)
+        call_node = synthesizer.call(function_node, args, kwargs, callback)
         synthesizer.finalize()
         self.typedtree.append(call_node)
 
diff --git a/artiq/compiler/transforms/llvm_ir_generator.py b/artiq/compiler/transforms/llvm_ir_generator.py
index 127fe32c0..b563476e1 100644
--- a/artiq/compiler/transforms/llvm_ir_generator.py
+++ b/artiq/compiler/transforms/llvm_ir_generator.py
@@ -907,9 +907,12 @@ class LLVMIRGenerator:
 
         llargs = []
         for arg in args:
-            llarg = self.map(arg)
-            llargslot = self.llbuilder.alloca(llarg.type)
-            self.llbuilder.store(llarg, llargslot)
+            if builtins.is_none(arg.type):
+                llargslot = self.llbuilder.alloca(ll.LiteralStructType([]))
+            else:
+                llarg = self.map(arg)
+                llargslot = self.llbuilder.alloca(llarg.type)
+                self.llbuilder.store(llarg, llargslot)
             llargs.append(llargslot)
 
         self.llbuilder.call(self.llbuiltin("send_rpc"),
diff --git a/artiq/coredevice/core.py b/artiq/coredevice/core.py
index 217d1f3f3..c444577c5 100644
--- a/artiq/coredevice/core.py
+++ b/artiq/coredevice/core.py
@@ -56,12 +56,12 @@ class Core:
         self.core = self
         self.comm.core = self
 
-    def compile(self, function, args, kwargs, with_attr_writeback=True):
+    def compile(self, function, args, kwargs, set_result, with_attr_writeback=True):
         try:
             engine = diagnostic.Engine(all_errors_are_fatal=True)
 
             stitcher = Stitcher(engine=engine)
-            stitcher.stitch_call(function, args, kwargs)
+            stitcher.stitch_call(function, args, kwargs, set_result)
             stitcher.finalize()
 
             module = Module(stitcher, ref_period=self.ref_period)
@@ -76,7 +76,12 @@ class Core:
             raise CompileError(error.diagnostic) from error
 
     def run(self, function, args, kwargs):
-        object_map, kernel_library, symbolizer = self.compile(function, args, kwargs)
+        result = None
+        def set_result(new_result):
+            nonlocal result
+            result = new_result
+
+        object_map, kernel_library, symbolizer = self.compile(function, args, kwargs, set_result)
 
         if self.first_run:
             self.comm.check_ident()
@@ -87,6 +92,8 @@ class Core:
         self.comm.run()
         self.comm.serve(object_map, symbolizer)
 
+        return result
+
     @kernel
     def get_rtio_counter_mu(self):
         return rtio_get_counter()
diff --git a/artiq/test/coredevice/embedding.py b/artiq/test/coredevice/embedding.py
index a5a7095ad..04a136164 100644
--- a/artiq/test/coredevice/embedding.py
+++ b/artiq/test/coredevice/embedding.py
@@ -50,12 +50,10 @@ class DefaultArg(EnvExperiment):
         return foo
 
     @kernel
-    def run(self, callback):
-        callback(self.test())
+    def run(self):
+        return self.test()
 
 class DefaultArgTest(ExperimentCase):
     def test_default_arg(self):
         exp = self.create(DefaultArg)
-        def callback(value):
-            self.assertEqual(value, 42)
-        exp.run(callback)
+        self.assertEqual(exp.run(), 42)