diff --git a/aws/src/main/java/org/apache/iceberg/aws/s3/S3FileIO.java b/aws/src/main/java/org/apache/iceberg/aws/s3/S3FileIO.java
index e00356a94742..41d45db357f1 100644
--- a/aws/src/main/java/org/apache/iceberg/aws/s3/S3FileIO.java
+++ b/aws/src/main/java/org/apache/iceberg/aws/s3/S3FileIO.java
@@ -434,9 +434,9 @@ private Map<String, PrefixedS3Client> clientByPrefix() {
     if (null == clientByPrefix) {
       synchronized (this) {
         if (null == clientByPrefix) {
-          this.clientByPrefix = Maps.newHashMap();
+          Map<String, PrefixedS3Client> localClientByPrefix = Maps.newHashMap();
 
-          clientByPrefix.put(
+          localClientByPrefix.put(
               ROOT_PREFIX, new PrefixedS3Client(ROOT_PREFIX, properties, s3, s3Async));
           storageCredentials.stream()
               .filter(c -> c.prefix().startsWith(ROOT_PREFIX))
@@ -449,11 +449,12 @@ private Map<String, PrefixedS3Client> clientByPrefix() {
                             .putAll(storageCredential.config())
                             .buildKeepingLast();
 
-                    clientByPrefix.put(
+                    localClientByPrefix.put(
                         storageCredential.prefix(),
                         new PrefixedS3Client(
                             storageCredential.prefix(), propertiesWithCredentials, s3, s3Async));
                   });
+          this.clientByPrefix = localClientByPrefix;
         }
       }
     }
diff --git a/gcp/src/main/java/org/apache/iceberg/gcp/gcs/GCSFileIO.java b/gcp/src/main/java/org/apache/iceberg/gcp/gcs/GCSFileIO.java
index 0c76d1258c16..c77e17ad0894 100644
--- a/gcp/src/main/java/org/apache/iceberg/gcp/gcs/GCSFileIO.java
+++ b/gcp/src/main/java/org/apache/iceberg/gcp/gcs/GCSFileIO.java
@@ -193,9 +193,9 @@ private Map<String, PrefixedStorage> storageByPrefix() {
     if (null == storageByPrefix) {
       synchronized (this) {
         if (null == storageByPrefix) {
-          this.storageByPrefix = Maps.newHashMap();
+          Map<String, PrefixedStorage> localStorageByPrefix = Maps.newHashMap();
 
-          storageByPrefix.put(
+          localStorageByPrefix.put(
               ROOT_STORAGE_PREFIX,
               new PrefixedStorage(ROOT_STORAGE_PREFIX, properties, storageSupplier));
           storageCredentials.stream()
@@ -209,13 +209,14 @@ private Map<String, PrefixedStorage> storageByPrefix() {
                             .putAll(storageCredential.config())
                             .buildKeepingLast();
 
-                    storageByPrefix.put(
+                    localStorageByPrefix.put(
                         storageCredential.prefix(),
                         new PrefixedStorage(
                             storageCredential.prefix(),
                             propertiesWithCredentials,
                             storageSupplier));
                   });
+          this.storageByPrefix = localStorageByPrefix;
         }
       }
     }