diff --git a/ChangeLog.d/ssl_premaster_secret-empty.txt b/ChangeLog.d/ssl_premaster_secret-empty.txt
new file mode 100644
index 0000000000..0ce5f36ead
--- /dev/null
+++ b/ChangeLog.d/ssl_premaster_secret-empty.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Fix a compilation error on some platforms when including mbedtls/ssl.h
+     with all TLS support disabled. Fixes #6628.
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index e4d817fd74..f7ce059e24 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -619,6 +619,7 @@
 
 /* Dummy type used only for its size */
 union mbedtls_ssl_premaster_secret {
+    unsigned char dummy; /* Make the union non-empty even with SSL disabled */
 #if defined(MBEDTLS_KEY_EXCHANGE_RSA_ENABLED)
     unsigned char _pms_rsa[48];                         /* RFC 5246 8.1.1 */
 #endif