diff --git a/piecrust/src/session.rs b/piecrust/src/session.rs
index 0b81bdbe..e5e940c6 100644
--- a/piecrust/src/session.rs
+++ b/piecrust/src/session.rs
@@ -761,7 +761,7 @@ impl Session {
                 io: Arc::new(err),
             })?;
 
-        let arg_len = instance.write_bytes_to_arg_buffer(&fdata);
+        let arg_len = instance.write_bytes_to_arg_buffer(&fdata)?;
         instance.clear_arg_buffer_b();
         let ret_len = instance
             .call(fname, arg_len, limit)
@@ -779,7 +779,7 @@ impl Session {
             .map_err(Error::normalize)?;
         let ret = instance.read_bytes_from_arg_buffer(ret_len as u32);
         let ret_b = instance.read_from_arg_buffer_b();
-        let allowance = *ret_b.get(0).unwrap_or(&0u64);
+        let allowance = *ret_b.first().unwrap_or(&0u64);
         let charge = *ret_b.get(1).unwrap_or(&0u64);
 
         let mut spent = limit - instance.get_remaining_gas();