diff --git a/client/agent.c b/client/agent.c
index b804b50d..157b61c0 100644
--- a/client/agent.c
+++ b/client/agent.c
@@ -303,7 +303,7 @@ static struct l_dbus_message *request_user_password_method_call(
 
 	username_prompt = l_strdup_printf(COLOR_BLUE PROMPT_USERNAME " "
 						COLOR_OFF "%s\n", username);
-	display(username_prompt);
+	display("%s", username_prompt);
 	l_free(username_prompt);
 
 	display_agent_prompt(PROMPT_PASSWORD, true);
diff --git a/client/display.h b/client/display.h
index 2b0d8d7c..fbae0c67 100644
--- a/client/display.h
+++ b/client/display.h
@@ -32,8 +32,10 @@ struct command_family;
 #define CLEAR_SCREEN	"\033[2J"
 #define MARGIN		"  "
 
-void display(const char *format, ...);
-void display_table_header(const char *caption, const char *fmt, ...);
+void display(const char *format, ...)
+		__attribute__((format(printf, 1, 2)));
+void display_table_header(const char *caption, const char *fmt, ...)
+		__attribute__((format(printf, 2, 3)));
 void display_table_footer(void);
 void display_error(const char *error);
 void display_command_line(const char *command_family,