diff --git a/ChangeLog.d/fix-alpn-empty-list.txt b/ChangeLog.d/fix-alpn-empty-list.txt
new file mode 100644
index 0000000000..03b9345686
--- /dev/null
+++ b/ChangeLog.d/fix-alpn-empty-list.txt
@@ -0,0 +1,7 @@
+Bugfix
+   * Fix mbedtls_ssl_conf_alpn_protocols() silently accepting an empty
+     protocol list, which caused an invalid ALPN extension with a
+     zero-length protocol_name_list to be sent during the handshake,
+     violating RFC 7301 and causing conforming peers to reject the
+     connection with a fatal decode error. mbedtls_ssl_conf_alpn_protocols()
+     now returns MBEDTLS_ERR_SSL_BAD_INPUT_DATA for an empty list.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 0195576213..dbebdf9c57 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2526,6 +2526,16 @@ int mbedtls_ssl_conf_alpn_protocols(mbedtls_ssl_config *conf,
     size_t cur_len, tot_len;
     const char *const *p;
 
+    /*
+     * RFC 7301 3.1: the ALPN extension's protocol_name_list must contain
+     * at least one entry - an empty list is not a valid ProtocolNameList
+     * and must not be sent. Reject it here rather than silently sending
+     * an ALPN extension with a zero-length list later.
+     */
+    if (*protos == NULL) {
+        return MBEDTLS_ERR_SSL_BAD_INPUT_DATA;
+    }
+
     /*
      * RFC 7301 3.1: "Empty strings MUST NOT be included and byte strings
      * MUST NOT be truncated."