diff --git a/artiq/firmware/libboard_misoc/i2c.rs b/artiq/firmware/libboard_misoc/i2c.rs
index acade0c24..9e62a97ff 100644
--- a/artiq/firmware/libboard_misoc/i2c.rs
+++ b/artiq/firmware/libboard_misoc/i2c.rs
@@ -183,8 +183,11 @@ mod imp {
         if !write(busno, address << 1)? {
             return Err("PCA9548 failed to ack write address")
         }
-        if !write(busno, channels)? {
-            return Err("PCA9548 failed to ack control word")
+        // Duplicate control byte: one for SCL, one for SDA
+        for _ in 0..2 {
+            if !write(busno, channels)? {
+                return Err("PCA9548 failed to ack control word")
+            }
         }
         stop(busno)?;
         Ok(())