diff --git a/loopy/kernel/data.py b/loopy/kernel/data.py
index bdfadceeb6dee8c189662b8bbe297d1a3d9c7d0d..3ee3002b0cacf22cd2c9081cb5d8b3903c7c2dd5 100644
--- a/loopy/kernel/data.py
+++ b/loopy/kernel/data.py
@@ -266,7 +266,7 @@ class ImageArg(Record):
         return "<ImageArg '%s' of type %s>" % (self.name, self.dtype)
 
 class ValueArg(Record):
-    def __init__(self, name, dtype=None, approximately=None):
+    def __init__(self, name, dtype=None, approximately=1000):
         if dtype is not None:
             dtype = np.dtype(dtype)
 
@@ -277,7 +277,7 @@ class ValueArg(Record):
         return "<ValueArg '%s' of type %s>" % (self.name, self.dtype)
 
 class ScalarArg(ValueArg):
-    def __init__(self, name, dtype=None, approximately=None):
+    def __init__(self, name, dtype=None, approximately=1000):
         from warnings import warn
         warn("ScalarArg is a deprecated name of ValueArg",
                 DeprecationWarning, stacklevel=2)
diff --git a/loopy/preprocess.py b/loopy/preprocess.py
index f67b25e1fe8fb5cfb545d251e39ea046d428938b..85510831176b91d4d8e7de336f4663e7c31c7ee2 100644
--- a/loopy/preprocess.py
+++ b/loopy/preprocess.py
@@ -115,7 +115,7 @@ def infer_types_of_temporaries(kernel):
         if failed:
             if item is first_failure:
                 # this item has failed before, give up.
-                raise RuntimeError("could not determine type of '%s'" % item)
+                raise RuntimeError("could not determine type of '%s'" % item.name)
 
             if first_failure is None:
                 # remember the first failure for this round through the queue
@@ -592,10 +592,14 @@ def limit_boostability(kernel):
 def get_auto_axis_iname_ranking_by_stride(kernel, insn):
     from loopy.kernel.data import ImageArg, ValueArg
 
-    approximate_arg_values = dict(
-            (arg.name, arg.approximately)
-            for arg in kernel.args
-            if isinstance(arg, ValueArg))
+    approximate_arg_values = {}
+    for arg in kernel.args:
+        if isinstance(arg, ValueArg):
+            if arg.approximately is not None:
+                approximate_arg_values[arg.name] = arg.approximately
+            else:
+                raise RuntimeError("No approximate arg value specified for '%s'"
+                        % arg.name)
 
     # {{{ find all array accesses in insn