diff --git a/infer/annotations/com/facebook/infer/annotation/AssumeThreadSafe.java b/infer/annotations/com/facebook/infer/annotation/AssumeThreadSafe.java
index 7e0f51e3c..e2f8ca64b 100644
--- a/infer/annotations/com/facebook/infer/annotation/AssumeThreadSafe.java
+++ b/infer/annotations/com/facebook/infer/annotation/AssumeThreadSafe.java
@@ -20,4 +20,5 @@ import java.lang.annotation.Target;
 @Target({ ElementType.METHOD })
 @Retention(RetentionPolicy.CLASS)
 public @interface AssumeThreadSafe {
+  String because(); /** describe why the thread-safety assumption is reasonable */
 }
diff --git a/infer/tests/codetoanalyze/java/threadsafety/Annotations.java b/infer/tests/codetoanalyze/java/threadsafety/Annotations.java
index 3fbabb303..c34526b3d 100644
--- a/infer/tests/codetoanalyze/java/threadsafety/Annotations.java
+++ b/infer/tests/codetoanalyze/java/threadsafety/Annotations.java
@@ -89,7 +89,7 @@ class Annotations {
     this.f = new Object();
   }
 
-  @AssumeThreadSafe
+  @AssumeThreadSafe(because = "it's a test")
   public void assumeThreadSafeOk() {
     this.f = new Object();
   }