diff --git a/compat.c b/compat.c

index 46dfe3a9c2e..478a9403eea 100644

--- a/compat.c

+++ b/compat.c

@@ -190,26 +190,26 @@ compat_pkalg_proposal(struct ssh *ssh, char *pkalg_prop)

 char *

 compat_kex_proposal(struct ssh *ssh, char *p)

 {

-	char *cp = NULL;

+	char *cp = NULL, *cp2 = NULL;

 	if ((ssh->compat & (SSH_BUG_CURVE25519PAD|SSH_OLD_DHGEX)) == 0)

 		return xstrdup(p);

 	debug2_f("original KEX proposal: %s", p);

 	if ((ssh->compat & SSH_BUG_CURVE25519PAD) != 0)

-		if ((p = match_filter_denylist(p,

+		if ((cp = match_filter_denylist(p,

 		    "curve25519-sha256@libssh.org")) == NULL)

 			fatal("match_filter_denylist failed");

 	if ((ssh->compat & SSH_OLD_DHGEX) != 0) {

-		cp = p;

-		if ((p = match_filter_denylist(p,

+		if ((cp2 = match_filter_denylist(cp ? cp : p,

 		    "diffie-hellman-group-exchange-sha256,"

 		    "diffie-hellman-group-exchange-sha1")) == NULL)

 			fatal("match_filter_denylist failed");

 		free(cp);

+		cp = cp2;

 	}

-	debug2_f("compat KEX proposal: %s", p);

-	if (*p == '\0')

+	if (cp == NULL || *cp == '\0')

 		fatal("No supported key exchange algorithms found");

-	return p;

+	debug2_f("compat KEX proposal: %s", cp);

+	return cp;

 }