diff --git a/virtio-devices/src/rng.rs b/virtio-devices/src/rng.rs
index 227b2a213..eae688ef0 100644
--- a/virtio-devices/src/rng.rs
+++ b/virtio-devices/src/rng.rs
@@ -230,6 +230,10 @@ impl Rng {
 }
 
 impl VirtioDevice for Rng {
+    fn config_size(&self) -> Option<u64> {
+        Some(0)
+    }
+
     fn device_type(&self) -> u32 {
         self.common.device_type
     }
diff --git a/virtio-devices/src/rtc.rs b/virtio-devices/src/rtc.rs
index 57cb73cf3..540508e5e 100644
--- a/virtio-devices/src/rtc.rs
+++ b/virtio-devices/src/rtc.rs
@@ -601,6 +601,10 @@ impl Rtc {
 }
 
 impl VirtioDevice for Rtc {
+    fn config_size(&self) -> Option<u64> {
+        Some(0)
+    }
+
     fn device_type(&self) -> u32 {
         self.common.device_type
     }
diff --git a/virtio-devices/src/watchdog.rs b/virtio-devices/src/watchdog.rs
index 220a5359a..01619f79d 100644
--- a/virtio-devices/src/watchdog.rs
+++ b/virtio-devices/src/watchdog.rs
@@ -310,6 +310,10 @@ fn timerfd_setup(timer: &File, secs: i64) -> Result<(), io::Error> {
 }
 
 impl VirtioDevice for Watchdog {
+    fn config_size(&self) -> Option<u64> {
+        Some(0)
+    }
+
     fn device_type(&self) -> u32 {
         self.common.device_type
     }