diff --git a/cmd-set-option.c b/cmd-set-option.c
index 356668ee..491353a8 100644
--- a/cmd-set-option.c
+++ b/cmd-set-option.c
@@ -227,10 +227,11 @@ enum cmd_retval
 cmd_set_option_user(struct cmd *self, struct cmd_q *cmdq, const char *optstr,
     const char *valstr)
 {
-	struct args	*args = self->args;
-	struct session	*s = cmdq->state.tflag.s;
-	struct winlink	*wl = cmdq->state.tflag.wl;
-	struct options	*oo;
+	struct args		*args = self->args;
+	struct session		*s = cmdq->state.tflag.s;
+	struct winlink		*wl = cmdq->state.tflag.wl;
+	struct options		*oo;
+	struct options_entry	*o;
 
 	if (args_has(args, 's'))
 		oo = global_options;
@@ -262,18 +263,22 @@ cmd_set_option_user(struct cmd *self, struct cmd_q *cmdq, const char *optstr,
 		}
 		options_remove(oo, optstr);
 	} else {
-		if (valstr == NULL) {
-			cmdq_error(cmdq, "empty value");
-			return (CMD_RETURN_ERROR);
-		}
-		if (args_has(args, 'o') && options_find1(oo, optstr) != NULL) {
+		o = options_find1(oo, optstr);
+		if (args_has(args, 'o') && o != NULL) {
 			if (!args_has(args, 'q')) {
 				cmdq_error(cmdq, "already set: %s", optstr);
 				return (CMD_RETURN_ERROR);
 			}
 			return (CMD_RETURN_NORMAL);
 		}
-		options_set_string(oo, optstr, "%s", valstr);
+		if (valstr == NULL) {
+			cmdq_error(cmdq, "empty value");
+			return (CMD_RETURN_ERROR);
+		}
+		if (o != NULL && args_has(args, 'a'))
+			options_set_string(oo, optstr, "%s%s", o->str, valstr);
+		else
+			options_set_string(oo, optstr, "%s", valstr);
 	}
 	return (CMD_RETURN_NORMAL);
 }
diff --git a/options.c b/options.c
index df79ac4b..cd3bd8ec 100644
--- a/options.c
+++ b/options.c
@@ -128,19 +128,23 @@ options_set_string(struct options *oo, const char *name, const char *fmt, ...)
 {
 	struct options_entry	*o;
 	va_list			 ap;
+	char			*s;
 
+	s = NULL;
 	if ((o = options_find1(oo, name)) == NULL) {
 		o = xmalloc(sizeof *o);
 		o->name = xstrdup(name);
 		RB_INSERT(options_tree, &oo->tree, o);
 		memcpy(&o->style, &grid_default_cell, sizeof o->style);
 	} else if (o->type == OPTIONS_STRING)
-		free(o->str);
+		s = o->str;
 
 	va_start(ap, fmt);
 	o->type = OPTIONS_STRING;
 	xvasprintf(&o->str, fmt, ap);
 	va_end(ap);
+
+	free(s);
 	return (o);
 }