diff --git a/mongoose.c b/mongoose.c
index f0bd66ea..a4a564a5 100644
--- a/mongoose.c
+++ b/mongoose.c
@@ -4865,13 +4865,14 @@ static void read_conn(struct mg_connection *c, struct pkt *pkt) {
     tx_tcp(c->mgr->ifp, s->mac, rem_ip, flags, c->loc.port, c->rem.port,
            mg_htonl(s->seq), mg_htonl(s->ack), "", 0);
     if (pkt->pay.len == 0) return;  // if no data, we're done
-  } else if (pkt->pay.len == 0) {   // this is an ACK
+  } else if (pkt->pay.len <= 1 && mg_ntohl(pkt->tcp->seq) == s->ack - 1) {
+    // Keep-Alive (RFC-9293 3.8.4, allow erroneous implementations)
+    MG_VERBOSE(("%lu keepalive ACK", c->id));
+    tx_tcp(c->mgr->ifp, s->mac, rem_ip, TH_ACK, c->loc.port, c->rem.port,
+           mg_htonl(s->seq), mg_htonl(s->ack), NULL, 0);
+    return;                        // no data to process
+  } else if (pkt->pay.len == 0) {  // this is an ACK
     if (s->fin_rcvd && s->ttype == MIP_TTYPE_FIN) s->twclosure = true;
-    if (mg_ntohl(pkt->tcp->seq) == s->ack - 1) { // this is a keepalive probe
-      MG_VERBOSE(("%lu keepalive ACK", c->id));
-      tx_tcp(c->mgr->ifp, s->mac, rem_ip, TH_ACK, c->loc.port, c->rem.port,
-             mg_htonl(s->seq), mg_htonl(s->ack), NULL, 0);
-    }
     return;  // no data to process
   } else if (seq != s->ack) {
     uint32_t ack = (uint32_t) (mg_htonl(pkt->tcp->seq) + pkt->pay.len);
diff --git a/src/net_builtin.c b/src/net_builtin.c
index 3780292a..37b64b51 100644
--- a/src/net_builtin.c
+++ b/src/net_builtin.c
@@ -701,13 +701,14 @@ static void read_conn(struct mg_connection *c, struct pkt *pkt) {
     tx_tcp(c->mgr->ifp, s->mac, rem_ip, flags, c->loc.port, c->rem.port,
            mg_htonl(s->seq), mg_htonl(s->ack), "", 0);
     if (pkt->pay.len == 0) return;  // if no data, we're done
-  } else if (pkt->pay.len == 0) {   // this is an ACK
+  } else if (pkt->pay.len <= 1 && mg_ntohl(pkt->tcp->seq) == s->ack - 1) {
+    // Keep-Alive (RFC-9293 3.8.4, allow erroneous implementations)
+    MG_VERBOSE(("%lu keepalive ACK", c->id));
+    tx_tcp(c->mgr->ifp, s->mac, rem_ip, TH_ACK, c->loc.port, c->rem.port,
+           mg_htonl(s->seq), mg_htonl(s->ack), NULL, 0);
+    return;                        // no data to process
+  } else if (pkt->pay.len == 0) {  // this is an ACK
     if (s->fin_rcvd && s->ttype == MIP_TTYPE_FIN) s->twclosure = true;
-    if (mg_ntohl(pkt->tcp->seq) == s->ack - 1) { // this is a keepalive probe
-      MG_VERBOSE(("%lu keepalive ACK", c->id));
-      tx_tcp(c->mgr->ifp, s->mac, rem_ip, TH_ACK, c->loc.port, c->rem.port,
-             mg_htonl(s->seq), mg_htonl(s->ack), NULL, 0);
-    }
     return;  // no data to process
   } else if (seq != s->ack) {
     uint32_t ack = (uint32_t) (mg_htonl(pkt->tcp->seq) + pkt->pay.len);