diff --git a/src/format/io.rs b/src/format/io.rs
index 271be16..e754bd0 100644
--- a/src/format/io.rs
+++ b/src/format/io.rs
@@ -89,7 +89,7 @@ where
         Err(err) => err
             .raw_os_error()
             .map(|code| unsafe { crate::ffw_error_from_posix(code as _) })
-            .unwrap_or(unsafe { crate::ffw_error_unknown() }) as i64,
+            .unwrap_or_else(|| unsafe { crate::ffw_error_unknown() }) as i64,
     }
 }