diff --git a/sim/src/api.rs b/sim/src/api.rs
index 71d46430..ec502a2d 100644
--- a/sim/src/api.rs
+++ b/sim/src/api.rs
@@ -29,6 +29,9 @@ pub extern fn sim_flash_write(dev: *mut Flash, offset: u32, src: *const u8, size
 fn map_err(err: Result<()>) -> libc::c_int {
     match err {
         Ok(()) => 0,
-        Err(_) => -1,
+        Err(e) => {
+            warn!("{}", e);
+            -1
+        },
     }
 }
diff --git a/sim/src/flash.rs b/sim/src/flash.rs
index 797a044d..adbb69a3 100644
--- a/sim/src/flash.rs
+++ b/sim/src/flash.rs
@@ -94,8 +94,10 @@ impl Flash {
         }
 
         let mut sub = &mut self.data[offset .. offset + payload.len()];
-        if sub.iter().any(|x| *x != 0xFF) {
-            bail!(ewrite(format!("Write to non-FF location: offset: {:x}", offset)));
+        for (i, x) in sub.iter().enumerate() {
+            if *x != 0xFF {
+                bail!(ewrite(format!("Write to non-FF location at 0x{:x}", offset + i)));
+            }
         }
 
         sub.copy_from_slice(payload);