diff --git a/sim/src/image.rs b/sim/src/image.rs
index 60369ef..d183c40 100644
--- a/sim/src/image.rs
+++ b/sim/src/image.rs
@@ -84,7 +84,7 @@
     /// Construct a new image builder for the given device.  Returns
     /// Some(builder) if is possible to test this configuration, or None if
     /// not possible (for example, if there aren't enough image slots).
-    pub fn new(device: DeviceName, align: u8, erased_val: u8) -> Option<Self> {
+    pub fn new(device: DeviceName, align: usize, erased_val: u8) -> Option<Self> {
         let (flash, areadesc) = Self::make_device(device, align, erased_val);
 
         let num_images = Caps::get_num_images();
@@ -227,7 +227,7 @@
     }
 
     /// Build the Flash and area descriptor for a given device.
-    pub fn make_device(device: DeviceName, align: u8, erased_val: u8) -> (SimMultiFlash, AreaDesc) {
+    pub fn make_device(device: DeviceName, align: usize, erased_val: u8) -> (SimMultiFlash, AreaDesc) {
         match device {
             DeviceName::Stm32f4 => {
                 // STM style flash.  Large sectors, with a large scratch area.
diff --git a/sim/src/lib.rs b/sim/src/lib.rs
index bb88bf3..fda376a 100644
--- a/sim/src/lib.rs
+++ b/sim/src/lib.rs
@@ -80,7 +80,7 @@
 }
 
 #[derive(Debug)]
-struct AlignArg(u8);
+struct AlignArg(usize);
 
 struct AlignArgVisitor;
 
@@ -91,11 +91,11 @@
         formatter.write_str("1, 2, 4 or 8")
     }
 
-    fn visit_u8<E>(self, n: u8) -> Result<Self::Value, E>
+    fn visit_u32<E>(self, n: u32) -> Result<Self::Value, E>
         where E: serde::de::Error
     {
         Ok(match n {
-            1 | 2 | 4 | 8 => AlignArg(n),
+            1 | 2 | 4 | 8 => AlignArg(n as usize),
             n => {
                 let err = format!("Could not deserialize '{}' as alignment", n);
                 return Err(E::custom(err));
@@ -169,7 +169,7 @@
         }
     }
 
-    pub fn run_single(&mut self, device: DeviceName, align: u8, erased_val: u8) {
+    pub fn run_single(&mut self, device: DeviceName, align: usize, erased_val: u8) {
         warn!("Running on device {} with alignment {}", device, align);
 
         let run = match ImagesBuilder::new(device, align, erased_val) {