diff --git a/artiq/firmware/ksupport/rtio.rs b/artiq/firmware/ksupport/rtio.rs
index 682a118c6..0c95842ad 100644
--- a/artiq/firmware/ksupport/rtio.rs
+++ b/artiq/firmware/ksupport/rtio.rs
@@ -140,16 +140,17 @@ pub fn log(timestamp: i64, data: &[u8]) {
         for i in 0..data.len() {
             word <<= 8;
             word |= data[i] as u32;
-            if i != 0 && i % 4 == 0 {
+            if i % 4 == 3 {
                 rtio_o_data_write(0, word);
                 csr::rtio::o_we_write(1);
                 word = 0;
             }
         }
 
-        word <<= 8;
-        rtio_o_data_write(0, word);
-        csr::rtio::o_we_write(1);
+        if word != 0 {
+            rtio_o_data_write(0, word);
+            csr::rtio::o_we_write(1);
+        }
     }
 }