diff --git a/src/static-guarantees/state-machines.md b/src/static-guarantees/state-machines.md
index 5a27ffcf..84207911 100644
--- a/src/static-guarantees/state-machines.md
+++ b/src/static-guarantees/state-machines.md
@@ -68,7 +68,7 @@ impl GpioConfig {
     }
 
     pub fn set_direction(&mut self, is_output: bool) {
-        self.periph.modify(|r, w| {
+        self.periph.modify(|_r, w| {
             w.direction().set_bit(is_output)
         });
     }