diff --git a/include/libwebsockets/lws-spa.h b/include/libwebsockets/lws-spa.h
index c5ddab2c7..1045dc4a5 100644
--- a/include/libwebsockets/lws-spa.h
+++ b/include/libwebsockets/lws-spa.h
@@ -102,7 +102,8 @@ lws_spa_create(struct lws *wsi, const char * const *param_names,
 	       void *opt_data);
 
 typedef struct lws_spa_create_info {
-	const char * const *param_names; /* array of form parameter names, like "username" */
+	const char * const *param_names; /**< array of form parameter names, like "username"
+			These may be NULL, to make space for arbitrary POST items */
 	int count_params; /* count of param_names */
 	int max_storage; /* total amount of form parameter values we can store */
 	lws_spa_fileupload_cb opt_cb; /* NULL, or callback to receive file upload data. */
diff --git a/lib/roles/http/server/lws-spa.c b/lib/roles/http/server/lws-spa.c
index a39c66138..990c360c1 100644
--- a/lib/roles/http/server/lws-spa.c
+++ b/lib/roles/http/server/lws-spa.c
@@ -1,7 +1,7 @@
 /*
  * libwebsockets - small server side websockets and web server implementation
  *
- * Copyright (C) 2010 - 2019 Andy Green <andy@warmcat.com>
+ * Copyright (C) 2010 - 2021 Andy Green <andy@warmcat.com>
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to
@@ -482,7 +482,23 @@ lws_urldecode_spa_lookup(struct lws_spa *spa, const char *name)
 	int n;
 
 	for (n = 0; n < spa->i.count_params; n++) {
-		if (!strcmp(*pp, name))
+		if (!*pp && !spa->i.param_names_stride && spa->i.ac) {
+			unsigned int len = (unsigned int)strlen(name);
+			char **ptr = (char**)spa->i.param_names;
+
+			/* Use NULLs at end of list to dynamically create
+			 * unknown entries */
+
+			ptr[n] = lwsac_use(spa->i.ac, len + 1, spa->i.ac_chunk_size);
+			if (!ptr[n])
+				return -1;
+
+			memcpy(ptr[n], name, len);
+			ptr[n][len] = '\0';
+
+			return n;
+		}
+		if (*pp && !strcmp(*pp, name))
 			return n;
 
 		if (spa->i.param_names_stride)
diff --git a/minimal-examples-lowlevel/http-server/minimal-http-server-form-post-lwsac/minimal-http-server-form-post.c b/minimal-examples-lowlevel/http-server/minimal-http-server-form-post-lwsac/minimal-http-server-form-post.c
index 225cdd619..b4e16064b 100644
--- a/minimal-examples-lowlevel/http-server/minimal-http-server-form-post-lwsac/minimal-http-server-form-post.c
+++ b/minimal-examples-lowlevel/http-server/minimal-http-server-form-post-lwsac/minimal-http-server-form-post.c
@@ -27,9 +27,13 @@ struct pss {
 
 static int interrupted;
 
-static const char * const param_names[] = {
+/* get 2 parameters at predefined pos and up to 3 unknown parameters */
+static const char * param_names[] = {
 	"text1",
 	"send",
+	NULL,
+	NULL,
+	NULL
 };
 
 enum enum_param_names {
@@ -98,6 +102,8 @@ callback_http(struct lws *wsi, enum lws_callback_reasons reason, void *user,
 		/* we just dump the decoded things to the log */
 
 		for (n = 0; n < (int)LWS_ARRAY_SIZE(param_names); n++) {
+			if (!param_names[n])
+				break;
 			if (!lws_spa_get_string(pss->spa, n))
 				lwsl_user("%s: undefined\n", param_names[n]);
 			else
@@ -107,8 +113,6 @@ callback_http(struct lws *wsi, enum lws_callback_reasons reason, void *user,
 				    lws_spa_get_string(pss->spa, n));
 		}
 
-		lwsac_free(&pss->ac);
-
 		/*
 		 * Our response is to redirect to a static page.  We could
 		 * have generated a dynamic html page here instead.
@@ -123,10 +127,9 @@ callback_http(struct lws *wsi, enum lws_callback_reasons reason, void *user,
 	case LWS_CALLBACK_HTTP_DROP_PROTOCOL:
 		/* called when our wsi user_space is going to be destroyed */
 		if (pss->spa) {
-			lws_spa_destroy(pss->spa);
+			lws_spa_destroy(pss->spa); /* lwsac_free called this */
 			pss->spa = NULL;
 		}
-		lwsac_free(&pss->ac);
 		break;
 
 	default: