diff --git a/capnp/lib/capnp.pyx b/capnp/lib/capnp.pyx
index 71262c6..188a151 100644
--- a/capnp/lib/capnp.pyx
+++ b/capnp/lib/capnp.pyx
@@ -1526,6 +1526,17 @@ cdef class _VoidPromise:
         if self.is_consumed:
             raise RuntimeError('Promise was already used in a consuming operation. You can no longer use this Promise object')
 
+        argspec = None
+        try:
+            argspec = _inspect.getargspec(func)
+        except:
+            pass
+        if argspec:
+            args_length = len(argspec.args) if argspec.args else 0
+            defaults_length = len(argspec.defaults) if argspec.defaults else 0
+            if args_length - defaults_length != 0:
+                raise ValueError('Function passed to `then` call must take no arguments')
+
         return Promise()._init(helpers.then(deref(self.thisptr), <PyObject *>func, <PyObject *>error_func).attach(capnp.makePyRefCounter(<PyObject *>func), capnp.makePyRefCounter(<PyObject *>error_func)), self)
 
     cpdef as_pypromise(self) except +reraise_kj_exception:
@@ -1587,6 +1598,17 @@ cdef class _RemotePromise:
         if self.is_consumed:
             raise RuntimeError('Promise was already used in a consuming operation. You can no longer use this Promise object')
 
+        argspec = None
+        try:
+            argspec = _inspect.getargspec(func)
+        except:
+            pass
+        if argspec:
+            args_length = len(argspec.args) if argspec.args else 0
+            defaults_length = len(argspec.defaults) if argspec.defaults else 0
+            if args_length - defaults_length != 1:
+                raise ValueError('Function passed to `then` call must take exactly one argument')
+
         Py_INCREF(func)
         Py_INCREF(error_func)
 
@@ -1656,16 +1678,21 @@ cpdef join_promises(promises) except +reraise_kj_exception:
 
 cdef class _Request(_DynamicStructBuilder):
     cdef Request * thisptr_child
+    cdef public bint is_consumed
 
     cdef _init_child(self, Request other, parent):
         self.thisptr_child = new Request(moveRequest(other))
         self._init(<DynamicStruct_Builder>deref(self.thisptr_child), parent)
+        self.is_consumed = False
         return self
 
     def __dealloc__(self):
         del self.thisptr_child
 
     cpdef send(self):
+        if self.is_consumed:
+            raise ValueError('Request has already been sent. You can only send a request once.')
+        self.is_consumed = True
         return _RemotePromise()._init(self.thisptr_child.send(), self._parent)
 
 cdef class _Response(_DynamicStructReader):
diff --git a/test/test_capability.py b/test/test_capability.py
index 7278f8d..5cd09a2 100644
--- a/test/test_capability.py
+++ b/test/test_capability.py
@@ -291,6 +291,17 @@ def test_timer():
         joined.wait()
 
 
+def test_double_send():
+    client = capability.TestInterface._new_client(Server())
+
+    req = client._request('foo')
+    req.i = 5
+
+    req.send()
+    with pytest.raises(ValueError):
+        req.send()
+
+
 def test_then_args():
     capnp.Promise(0).then(lambda x: 1)
 
@@ -299,3 +310,18 @@ def test_then_args():
 
     with pytest.raises(ValueError):
         capnp.Promise(0).then(lambda x, y: 1)
+
+    capnp.getTimer().after_delay(1).then(lambda: 1)  # after_delay is a VoidPromise
+
+    with pytest.raises(ValueError):
+        capnp.getTimer().after_delay(1).then(lambda x: 1)
+
+    client = capability.TestInterface._new_client(Server())
+
+    client.foo(i=5).then(lambda x: 1)
+
+    with pytest.raises(ValueError):
+        client.foo(i=5).then(lambda: 1)
+
+    with pytest.raises(ValueError):
+        client.foo(i=5).then(lambda x, y: 1)