diff --git a/loopy/compiled.py b/loopy/compiled.py
index 2859a20476e20b8f9bfe059ff82a0cc47f4dcfb6..3f858536745f04e74e5c28760bce942645112cb4 100644
--- a/loopy/compiled.py
+++ b/loopy/compiled.py
@@ -370,8 +370,7 @@ def fill_rand(ary):
 class TestArgInfo(Record):
     pass
 
-def make_ref_args(kernel, queue, parameters,
-        fill_value):
+def make_ref_args(kernel, queue, parameters, fill_value):
     from loopy.kernel.data import ValueArg, GlobalArg, ImageArg
 
     from pymbolic import evaluate
@@ -420,10 +419,17 @@ def make_ref_args(kernel, queue, parameters,
                 alloc_size = sum(astrd*(alen-1)
                         for alen, astrd in zip(shape, strides)) + 1
 
-                itemsize = arg.dtype.itemsize
+                dtype = arg.dtype
+                if dtype is None:
+                    raise RuntimeError("dtype for argument '%s' is not yet "
+                            "known. Perhaps you want to use loopy.add_argument_dtypes "
+                            "or loopy.infer_argument_dtypes?"
+                            % arg.name)
+
+                itemsize = dtype.itemsize
                 numpy_strides = [itemsize*s for s in strides]
 
-                storage_array = cl_array.empty(queue, alloc_size, arg.dtype)
+                storage_array = cl_array.empty(queue, alloc_size, dtype)
                 ary = cl_array.as_strided(storage_array, shape, numpy_strides)
 
             if is_output:
@@ -431,11 +437,11 @@ def make_ref_args(kernel, queue, parameters,
                     raise RuntimeError("write-mode images not supported in "
                             "automatic testing")
 
-                if arg.dtype.isbuiltin:
+                if dtype.isbuiltin:
                     storage_array.fill(fill_value)
                 else:
                     from warnings import warn
-                    warn("Cannot pre-fill array of dtype '%s'" % arg.dtype)
+                    warn("Cannot pre-fill array of dtype '%s'" % dtype)
 
                 ref_args[arg.name] = ary
             else:
@@ -619,7 +625,8 @@ def auto_test_vs_ref(ref_knl, ctx, kernel_gen, op_count=[], op_label=[], paramet
         print_ref_code=False, print_code=True, warmup_rounds=2,
         edit_code=False, dump_binary=False, codegen_kwargs={},
         options=[],
-        fills_entire_output=True, do_check=True, check_result=None):
+        fills_entire_output=True, do_check=True, check_result=None
+        ):
     """Compare results of `ref_knl` to the kernels generated by the generator
     `kernel_gen`.