diff --git a/b2/boom.c b/b2/boom.c
index afbd3bf..68c5239 100644
--- a/b2/boom.c
+++ b/b2/boom.c
@@ -136,6 +136,7 @@ int main(int argc, char **argv)
 	int i;
 
 	dollar = unique("$");
+	subst_init();
 	subex_init();
 	for (i = 1; i != argc; i++) {
 		if (*argv[i] != '-') {
diff --git a/b2/subex.c b/b2/subex.c
index db886f2..b073afb 100644
--- a/b2/subex.c
+++ b/b2/subex.c
@@ -29,7 +29,7 @@
 #define FIELDS	10	/* fields in KiCad schematics */
 
 
-static const char *fn = NULL, *f[FIELDS];
+static const char *f[FIELDS];
 
 /* Jump targets that can never be reached. */
 static struct subst jump_end;
@@ -248,7 +248,6 @@ void subex_init(void)
 	int i;
 	char tmp[4];
 
-	fn = unique("FN");
 	for (i = 0; i != FIELDS; i++) {
 		sprintf(tmp, "F%d", i);
 		f[i] = unique(tmp);
diff --git a/b2/subst.c b/b2/subst.c
index 0774801..ff4d7be 100644
--- a/b2/subst.c
+++ b/b2/subst.c
@@ -25,6 +25,9 @@
 #include "subst.h"
 
 
+const char *fn;
+
+
 /* ----- Rule set construction --------------------------------------------- */
 
 
@@ -297,7 +300,7 @@ static int find_var_use(const char *var, const struct subst *sub)
 	while (sub) {
 		switch (sub->type) {
 		case st_match:
-			if (sub->u.match.src == var)
+			if (sub->u.match.src == var && var != fn)
 				return 1;
 			break;
 		case st_assign:
@@ -459,3 +462,9 @@ void subst_dump(FILE *file, const struct subst *sub)
 {
 	recurse_dump(file, sub, 0);
 }
+
+
+void subst_init(void)
+{
+	fn = unique("FN");
+}
diff --git a/b2/subst.h b/b2/subst.h
index 3a3efbb..0406df3 100644
--- a/b2/subst.h
+++ b/b2/subst.h
@@ -73,6 +73,9 @@ struct subst {
 #define	MULT_CHARS	"GMkmunpf"
 
 
+extern const char *fn;
+
+
 struct subst *subst_match(const char *src, const char *re);
 struct subst *subst_assign(const char *dst, enum relop op, const char *pat);
 struct subst *subst_end(void);
@@ -84,4 +87,6 @@ void subst_finalize(struct subst *sub);
 
 void subst_dump(FILE *file, const struct subst *sub);
 
+void subst_init(void);
+
 #endif /* !SUBST_H */