diff --git a/wsme/rest/args.py b/wsme/rest/args.py
index 1492222..4574cdd 100644
--- a/wsme/rest/args.py
+++ b/wsme/rest/args.py
@@ -171,10 +171,14 @@ def args_from_args(funcdef, args, kwargs):
         try:
             newargs.append(from_param(argdef.datatype, arg))
         except Exception:
+            if isinstance(argdef.datatype, UserType):
+                datatype_name = argdef.datatype.name
+            else:
+                datatype_name = argdef.datatype.__name__
             raise InvalidInput(
                 argdef.name,
                 arg,
-                "unable to convert to %s" % argdef.datatype.__name__)
+                "unable to convert to %s" % datatype_name)
     newkwargs = {}
     for argname, value in kwargs.items():
         newkwargs[argname] = from_param(
diff --git a/wsme/tests/test_protocols_commons.py b/wsme/tests/test_protocols_commons.py
index b496de6..19908b2 100644
--- a/wsme/tests/test_protocols_commons.py
+++ b/wsme/tests/test_protocols_commons.py
@@ -3,7 +3,9 @@
 import datetime
 import unittest
 
-from wsme.rest.args import from_param, from_params
+from wsme.api import FunctionArgument, FunctionDefinition
+from wsme.rest.args import from_param, from_params, args_from_args
+from wsme.exc import InvalidInput
 
 from wsme.types import UserType, Unset, ArrayType, DictType
 
@@ -52,3 +54,24 @@ class TestProtocolsCommons(unittest.TestCase):
 
     def test_from_params_dict_unset(self):
         assert from_params(DictType(int, str), {}, 'a', set()) is Unset
+
+    def test_args_from_args_usertype(self):
+
+        class FakeType(UserType):
+            name = 'fake-type'
+            basetype = int
+
+        fake_type = FakeType()
+        fd = FunctionDefinition(FunctionDefinition)
+        fd.arguments.append(FunctionArgument('fake-arg', fake_type, True, 0))
+
+        new_args = args_from_args(fd, [1], {})
+        self.assertEqual([1], new_args[0])
+
+        # can't convert str to int
+        try:
+            args_from_args(fd, ['invalid-argument'], {})
+        except InvalidInput as e:
+            assert fake_type.name in str(e)
+        else:
+            self.fail('Should have thrown an InvalidInput')