diff --git a/loopy/kernel/tools.py b/loopy/kernel/tools.py
index 8bdc72d54a91c6e8b4f9ec0ca3053831627d3eae..15840180b1c19a4ad9edd31bd87a50ba9f946c48 100644
--- a/loopy/kernel/tools.py
+++ b/loopy/kernel/tools.py
@@ -1070,7 +1070,7 @@ def guess_var_shape(kernel, var_name):
 
             if n_axes == 1:
                 # Leave shape undetermined--we can live with that for 1D.
-                shape = (None,)
+                shape = None
             else:
                 raise LoopyError("cannot determine access range for '%s': "
                         "undetermined index in subscript(s) '%s'"
diff --git a/loopy/target/pyopencl_execution.py b/loopy/target/pyopencl_execution.py
index 2da25ba39ceef38a4af105913973226bd3773729..975c691a74d0d17bdca39243f515c5d04284893d 100644
--- a/loopy/target/pyopencl_execution.py
+++ b/loopy/target/pyopencl_execution.py
@@ -328,7 +328,8 @@ def generate_arg_setup(gen, kernel, implemented_data_info, options):
         # {{{ allocate written arrays, if needed
 
         if is_written and arg.arg_class in [lp.GlobalArg, lp.ConstantArg] \
-                and arg.shape is not None:
+                and arg.shape is not None \
+                and all(si is not None for si in arg.shape):
 
             if not isinstance(arg.dtype, NumpyType):
                 raise LoopyError("do not know how to pass arg of type '%s'"
diff --git a/test/test_loopy.py b/test/test_loopy.py
index 563964cf04dfbce5d8983b66010863ef36a74ce7..2c9425cc7d3e3cec21f197bbdb0253a0fa6f89df 100644
--- a/test/test_loopy.py
+++ b/test/test_loopy.py
@@ -518,6 +518,32 @@ def test_arg_guessing_with_reduction(ctx_factory):
     print(knl)
     print(lp.CompiledKernel(ctx, knl).get_highlighted_code())
 
+
+def test_unknown_arg_shape(ctx_factory):
+    ctx = ctx_factory()
+    from loopy.target.pyopencl import PyOpenCLTarget
+    from loopy.compiled import CompiledKernel
+    bsize = [256, 0]
+
+    knl = lp.make_kernel(
+        "{[i,j]: 0<=i<n and 0<=j<m}",
+        """
+        for i
+            <int32> gid = i/256
+            <int32> start = gid*256
+            for j
+                a[start + j] = a[start + j] + j
+            end
+        end
+        """,
+        seq_dependencies=True,
+        name="uniform_l",
+        target=PyOpenCLTarget(),
+        assumptions="m<=%d and m>=1 and n mod %d = 0" % (bsize[0], bsize[0]))
+
+    knl = lp.add_and_infer_dtypes(knl, dict(a=np.float32))
+    cl_kernel_info = CompiledKernel(ctx, knl).cl_kernel_info(frozenset())  # noqa
+
 # }}}