diff --git a/gax/src/main/java/com/google/api/gax/rpc/internal/ApiCallCustomContexts.java b/gax/src/main/java/com/google/api/gax/rpc/internal/ApiCallCustomContexts.java
index 002f968287..a054d804bf 100644
--- a/gax/src/main/java/com/google/api/gax/rpc/internal/ApiCallCustomContexts.java
+++ b/gax/src/main/java/com/google/api/gax/rpc/internal/ApiCallCustomContexts.java
@@ -39,7 +39,7 @@
 @InternalApi
 public final class ApiCallCustomContexts {
 
-  private ImmutableMap<Key, Object> customContexts;
+  private final ImmutableMap<Key, Object> customContexts;
 
   private ApiCallCustomContexts(ImmutableMap<Key, Object> contexts) {
     this.customContexts = Preconditions.checkNotNull(contexts);