diff --git a/src/bgzf/mod.rs b/src/bgzf/mod.rs
index e558d01f8..69fae4ed1 100644
--- a/src/bgzf/mod.rs
+++ b/src/bgzf/mod.rs
@@ -257,7 +257,7 @@ impl std::io::Write for Writer {
             htslib::bgzf_write(
                 self.inner,
                 buf.as_ptr() as *mut libc::c_void,
-                buf.len() as u64,
+                buf.len(),
             )
         };
         if nbytes < 0 {