diff --git a/coccinelle/run-coccinelle.sh b/coccinelle/run-coccinelle.sh
index 520de0ac42..4d882112e6 100755
--- a/coccinelle/run-coccinelle.sh
+++ b/coccinelle/run-coccinelle.sh
@@ -2,6 +2,7 @@
 
 top="$(git rev-parse --show-toplevel)"
 files="$(git ls-files ':/*.[ch]')"
+iso_defs="$top/coccinelle/systemd-definitions.iso"
 args=
 
 case "$1" in
@@ -21,7 +22,7 @@ for SCRIPT in ${@-$top/coccinelle/*.cocci} ; do
     TMPFILE=`mktemp`
     echo "+ spatch --sp-file $SCRIPT $args ..."
     parallel --halt now,fail=1 --keep-order --noswap --max-args=20 \
-             spatch --sp-file $SCRIPT $args ::: $files \
+             spatch --iso-file $iso_defs --sp-file $SCRIPT $args ::: $files \
              2>"$TMPFILE" || cat "$TMPFILE"
     echo -e "--x-- Processed $SCRIPT --x--\n"
 done
diff --git a/coccinelle/systemd-definitions.iso b/coccinelle/systemd-definitions.iso
new file mode 100644
index 0000000000..92db763a29
--- /dev/null
+++ b/coccinelle/systemd-definitions.iso
@@ -0,0 +1,20 @@
+/* Statement isomorphisms - replace explicit checks against NULL with a
+ * shorter variant, which relies on C's downgrade-to-bool feature.
+ * The expression metavariables should be declared as pointers, however,
+ * that doesn't work well with complex expressions like:
+ * if (UNIT(p)->default_dependencies != NULL)
+ */
+
+Statement
+@@
+expression X;
+statement S;
+@@
+if (X == NULL) S => if (!X) S
+
+Statement
+@@
+expression X;
+statement S;
+@@
+if (X != NULL) S => if (X) S