diff --git a/src/fontique/collection/mod.rs b/src/fontique/collection/mod.rs
index 69ecb6e..645349f 100644
--- a/src/fontique/collection/mod.rs
+++ b/src/fontique/collection/mod.rs
@@ -347,6 +347,8 @@ impl Inner {
             if let Some(families) = self.data.fallbacks.get(selector) {
                 self.fallback_cache.set(script, lang_key, families);
             } else if let Some(system) = self.system.as_ref() {
+                // Some platforms don't need mut System
+                #[allow(unused_mut)]
                 let mut system = system.fonts.lock().unwrap();
                 if let Some(family) = system.fallback(selector) {
                     self.data.fallbacks.set(selector, core::iter::once(family));