diff --git a/common.h b/common.h
index ff577c25..1182d77a 100644
--- a/common.h
+++ b/common.h
@@ -31,7 +31,7 @@ void ui_clear_key_queue();
 // Write a message to the on-screen log shown with Alt-L (also to stderr).
 // The screen is small, and users may need to report these messages to support,
 // so keep the output short and not too cryptic.
-void ui_print(const char *fmt, ...);
+void ui_print(const char *fmt, ...) __attribute__((format(printf, 1, 2)));
 
 // Display some header text followed by a menu of items, which appears
 // at the top of the screen (in place of any scrolling ui_print()
diff --git a/install.c b/install.c
index 37a4f077..35ba6ca7 100644
--- a/install.c
+++ b/install.c
@@ -136,7 +136,7 @@ try_update_binary(const char *path, ZipArchive *zip) {
         } else if (strcmp(command, "ui_print") == 0) {
             char* str = strtok(NULL, "\n");
             if (str) {
-                ui_print(str);
+                ui_print("%s", str);
             } else {
                 ui_print("\n");
             }