diff --git a/src/main/java/org/kohsuke/github/GHRepository.java b/src/main/java/org/kohsuke/github/GHRepository.java
index 32573ba9b3..5e8f192df0 100644
--- a/src/main/java/org/kohsuke/github/GHRepository.java
+++ b/src/main/java/org/kohsuke/github/GHRepository.java
@@ -113,11 +113,11 @@ public class GHRepository extends GHObject {
 
     private String pushed_at;
 
-    private Map<Integer, GHMilestone> milestones = new WeakHashMap<Integer, GHMilestone>();
+    private Map<Integer, GHMilestone> milestones = Collections.synchronizedMap(new WeakHashMap<>());
 
     private String default_branch, language;
 
-    private Map<String, GHCommit> commits = new WeakHashMap<String, GHCommit>();
+    private Map<String, GHCommit> commits = Collections.synchronizedMap(new WeakHashMap<>());
 
     @SkipFromToString
     private GHRepoPermission permissions;