diff --git a/embassy-stm32/build.rs b/embassy-stm32/build.rs
index 29af3c80..b766f073 100644
--- a/embassy-stm32/build.rs
+++ b/embassy-stm32/build.rs
@@ -213,7 +213,7 @@ fn main() {
         let region_type = format_ident!("{}", get_flash_region_type_name(region.name));
         flash_regions.extend(quote! {
             #[cfg(flash)]
-            pub struct #region_type<'d, MODE>(pub &'static crate::flash::FlashRegion, pub(crate) embassy_hal_common::PeripheralRef<'d, crate::peripherals::FLASH>, pub(crate) core::marker::PhantomData<MODE>);
+            pub struct #region_type<'d, MODE = crate::flash::Async>(pub &'static crate::flash::FlashRegion, pub(crate) embassy_hal_common::PeripheralRef<'d, crate::peripherals::FLASH>, pub(crate) core::marker::PhantomData<MODE>);
         });
     }
 
@@ -238,7 +238,7 @@ fn main() {
     let regions_len = flash_memory_regions.len();
     flash_regions.extend(quote! {
         #[cfg(flash)]
-        pub struct FlashLayout<'d, MODE> {
+        pub struct FlashLayout<'d, MODE = crate::flash::Async> {
             #(#fields),*,
             _mode: core::marker::PhantomData<MODE>,
         }
diff --git a/embassy-stm32/src/flash/f4.rs b/embassy-stm32/src/flash/f4.rs
index d50a35b4..3d696223 100644
--- a/embassy-stm32/src/flash/f4.rs
+++ b/embassy-stm32/src/flash/f4.rs
@@ -54,12 +54,12 @@ mod alt_regions {
         &ALT_BANK2_REGION3,
     ];
 
-    pub struct AltBank1Region3<'d, MODE>(pub &'static FlashRegion, PeripheralRef<'d, FLASH>, PhantomData<MODE>);
-    pub struct AltBank2Region1<'d, MODE>(pub &'static FlashRegion, PeripheralRef<'d, FLASH>, PhantomData<MODE>);
-    pub struct AltBank2Region2<'d, MODE>(pub &'static FlashRegion, PeripheralRef<'d, FLASH>, PhantomData<MODE>);
-    pub struct AltBank2Region3<'d, MODE>(pub &'static FlashRegion, PeripheralRef<'d, FLASH>, PhantomData<MODE>);
+    pub struct AltBank1Region3<'d, MODE = Async>(pub &'static FlashRegion, PeripheralRef<'d, FLASH>, PhantomData<MODE>);
+    pub struct AltBank2Region1<'d, MODE = Async>(pub &'static FlashRegion, PeripheralRef<'d, FLASH>, PhantomData<MODE>);
+    pub struct AltBank2Region2<'d, MODE = Async>(pub &'static FlashRegion, PeripheralRef<'d, FLASH>, PhantomData<MODE>);
+    pub struct AltBank2Region3<'d, MODE = Async>(pub &'static FlashRegion, PeripheralRef<'d, FLASH>, PhantomData<MODE>);
 
-    pub struct AltFlashLayout<'d, MODE> {
+    pub struct AltFlashLayout<'d, MODE = Async> {
         pub bank1_region1: Bank1Region1<'d, MODE>,
         pub bank1_region2: Bank1Region2<'d, MODE>,
         pub bank1_region3: AltBank1Region3<'d, MODE>,