diff --git a/b2/boom.c b/b2/boom.c
index 2cd6df4..afbd3bf 100644
--- a/b2/boom.c
+++ b/b2/boom.c
@@ -136,6 +136,7 @@ int main(int argc, char **argv)
 	int i;
 
 	dollar = unique("$");
+	subex_init();
 	for (i = 1; i != argc; i++) {
 		if (*argv[i] != '-') {
 			process(argv[i]);
diff --git a/b2/subex.c b/b2/subex.c
index b29ae6c..db886f2 100644
--- a/b2/subex.c
+++ b/b2/subex.c
@@ -235,18 +235,22 @@ static const struct subst *recurse_sub(const struct subst *sub,
 int substitute(const struct subst *sub, const struct param *in,
     struct param **out)
 {
-	int i;
-	char tmp[4];
 	enum subst_type cause = 0;
 
-	if (!fn) {
-		fn = unique("FN");
-		for (i = 0; i != FIELDS; i++) {
-			sprintf(tmp, "F%d", i);
-			f[i] = unique(tmp);
-		}
-	}
 	*out = NULL;
 	return recurse_sub(sub, in, NULL, NULL, NULL, NULL, out, &cause)
 	    != &jump_ignore;
 }
+
+
+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/subex.h b/b2/subex.h
index a664ae0..4fcfa27 100644
--- a/b2/subex.h
+++ b/b2/subex.h
@@ -20,4 +20,6 @@
 int substitute(const struct subst *sub, const struct param *in,
     struct param **out);
 
+void subex_init(void);
+
 #endif /* !SUBEX_H */