diff --git a/modules/random/src/main/java/org/stubit/random/RandomDuration.java b/modules/random/src/main/java/org/stubit/random/RandomDuration.java
index e031264..1e1728c 100644
--- a/modules/random/src/main/java/org/stubit/random/RandomDuration.java
+++ b/modules/random/src/main/java/org/stubit/random/RandomDuration.java
@@ -2,8 +2,10 @@
 
 import java.security.SecureRandom;
 import java.time.Duration;
+import org.jspecify.annotations.NullMarked;
 
 /** Generates random {@link Duration}s. */
+@NullMarked
 public class RandomDuration {
 
   private RandomDuration() {}