diff --git a/cmd-new-session.c b/cmd-new-session.c
index c47d66cd..666aeaac 100644
--- a/cmd-new-session.c
+++ b/cmd-new-session.c
@@ -102,6 +102,11 @@ cmd_new_session_exec(struct cmd *self, struct cmdq_item *item)
 	if (tmp != NULL) {
 		name = format_single(item, tmp, c, NULL, NULL, NULL);
 		newname = session_check_name(name);
+		if (newname == NULL) {
+			cmdq_error(item, "invalid session: %s", name);
+			free(name);
+			return (CMD_RETURN_ERROR);
+		}
 		free(name);
 	}
 	if (args_has(args, 'A')) {
@@ -134,8 +139,14 @@ cmd_new_session_exec(struct cmd *self, struct cmdq_item *item)
 			prefix = xstrdup(sg->name);
 		else if (groupwith != NULL)
 			prefix = xstrdup(groupwith->name);
-		else
+		else {
 			prefix = session_check_name(group);
+			if (prefix == NULL) {
+				cmdq_error(item, "invalid session group: %s",
+				    group);
+				goto fail;
+			}
+		}
 	}
 
 	/* Set -d if no client. */
diff --git a/cmd-rename-session.c b/cmd-rename-session.c
index 51b8ffc8..49fafe33 100644
--- a/cmd-rename-session.c
+++ b/cmd-rename-session.c
@@ -53,6 +53,11 @@ cmd_rename_session_exec(struct cmd *self, struct cmdq_item *item)
 
 	tmp = format_single_from_target(item, args->argv[0]);
 	newname = session_check_name(tmp);
+	if (newname == NULL) {
+		cmdq_error(item, "invalid session: %s", tmp);
+		free(tmp);
+		return (CMD_RETURN_ERROR);
+	}
 	free(tmp);
 	if (strcmp(newname, s->name) == 0) {
 		free(newname);
diff --git a/session.c b/session.c
index 93d50b47..cb0093a2 100644
--- a/session.c
+++ b/session.c
@@ -237,6 +237,8 @@ session_check_name(const char *name)
 {
 	char	*copy, *cp, *new_name;
 
+	if (*name == '\0')
+		return (NULL);
 	copy = xstrdup(name);
 	for (cp = copy; *cp != '\0'; cp++) {
 		if (*cp == ':' || *cp == '.')