diff --git a/loopy/cse.py b/loopy/cse.py
index 54618ed71fce020a5265d1541c47feb1599af205..624ed6b5e34526f50e4262639accc2b4ba1befee 100644
--- a/loopy/cse.py
+++ b/loopy/cse.py
@@ -473,7 +473,7 @@ def precompute(kernel, subst_use, dtype, sweep_inames=[],
             else:
                 return None
 
-        if subst_tag is None or subst_tag != tag:
+        if subst_tag is not None and subst_tag != tag:
             # use fall-back identity mapper
             return None
 
diff --git a/test/test_sem_tim.py b/test/test_sem_tim.py
index 0a6d40e47643a0e6163e34531a17d6d2fafa8903..8d1c03e7c9d7fa0775a39d3bca3b082bd1f28f30 100644
--- a/test/test_sem_tim.py
+++ b/test/test_sem_tim.py
@@ -7,6 +7,8 @@ import loopy as lp
 from pyopencl.tools import pytest_generate_tests_for_pyopencl \
         as pytest_generate_tests
 
+1/0 # inspect me
+
 
 
 
diff --git a/test/test_tim.py b/test/test_tim.py
index 316cbc9974b8ea91081fb677b2aa598a51288770..f5a8545d817a5fbc1f02f0a32efed8f70a342056 100644
--- a/test/test_tim.py
+++ b/test/test_tim.py
@@ -7,6 +7,8 @@ import loopy as lp
 from pyopencl.tools import pytest_generate_tests_for_pyopencl \
         as pytest_generate_tests
 
+1/0 # see sem_reagan?
+
 
 
 def test_tim2d(ctx_factory):