diff --git a/solr/modules/gcs-repository/src/test/org/apache/solr/gcs/GCSBackupRepositoryTest.java b/solr/modules/gcs-repository/src/test/org/apache/solr/gcs/GCSBackupRepositoryTest.java
index e4a2e084445..68c825777ae 100644
--- a/solr/modules/gcs-repository/src/test/org/apache/solr/gcs/GCSBackupRepositoryTest.java
+++ b/solr/modules/gcs-repository/src/test/org/apache/solr/gcs/GCSBackupRepositoryTest.java
@@ -45,6 +45,7 @@
 import org.apache.solr.common.util.NamedList;
 import org.apache.solr.core.backup.repository.BackupRepository;
 import org.junit.AfterClass;
+import org.junit.Assume;
 import org.junit.Test;
 
 /** Unit tests for {@link GCSBackupRepository} that use an in-memory Storage object */
@@ -123,7 +124,7 @@ public void testCopyIndexFileToHandlesZeroByteReads() throws Exception {
     GCSBackupRepository repo = createRepositoryWithStorage(realStorage);
     URI sourceDir = repo.resolve(getBaseUri(), "backup");
     BlobId blobId = BlobId.of(bucketName, sourceDir + "/source.dat");
-    realStorage.create(BlobInfo.newBuilder(blobId).build(), data);
+    createBlob(realStorage, blobId, data);
 
     Storage zeroReturningStorage = createZeroReturningStorage(realStorage);
     GCSBackupRepository proxyRepo = createRepositoryWithStorage(zeroReturningStorage);
@@ -150,7 +151,7 @@ public void testCopyIndexFileToCopiesFile() throws Exception {
     GCSBackupRepository repo = createRepositoryWithStorage(realStorage);
     URI sourceDir = repo.resolve(getBaseUri(), "backup");
     BlobId blobId = BlobId.of(bucketName, sourceDir + "/source.dat");
-    realStorage.create(BlobInfo.newBuilder(blobId).build(), data);
+    createBlob(realStorage, blobId, data);
 
     try (Directory dest = new ByteBuffersDirectory()) {
       repo.copyIndexFileTo(sourceDir, "source.dat", dest, "dest.dat");
@@ -163,6 +164,27 @@ public void testCopyIndexFileToCopiesFile() throws Exception {
     }
   }
 
+  /**
+   * Creates a blob, skipping (rather than failing) the test if the current default locale trips the
+   * known FakeStorageRpc/RFC3339 date-parsing bug - see {@link
+   * LocalStorageGCSBackupRepository#initializeBackupLocation()} for the same pattern.
+   */
+  private static void createBlob(Storage storage, BlobId blobId, byte[] data) {
+    try {
+      storage.create(BlobInfo.newBuilder(blobId).build(), data);
+    } catch (Exception e) {
+      final Throwable cause = e.getCause();
+      Assume.assumeFalse(
+          "This test uses a GCS mock library that is incompatible with the current default locale",
+          cause != null
+              && e instanceof StorageException
+              && cause.getMessage().contains("Invalid date/time format")
+              && cause instanceof NumberFormatException);
+      // Not the known locale incompatibility - a genuine failure, so don't swallow it.
+      throw new RuntimeException(e);
+    }
+  }
+
   /** Storage proxy that fails on {@code reader} so we can assert copy errors are propagated. */
   private static Storage createFailingStorage() {
     Storage delegate = LocalStorageHelper.customOptions(false).getService();
diff --git a/solr/modules/gcs-repository/src/test/org/apache/solr/gcs/LocalStorageGCSBackupRepository.java b/solr/modules/gcs-repository/src/test/org/apache/solr/gcs/LocalStorageGCSBackupRepository.java
index fc26a188f87..da54e135de4 100644
--- a/solr/modules/gcs-repository/src/test/org/apache/solr/gcs/LocalStorageGCSBackupRepository.java
+++ b/solr/modules/gcs-repository/src/test/org/apache/solr/gcs/LocalStorageGCSBackupRepository.java
@@ -96,13 +96,14 @@ protected void initializeBackupLocation() {
       createDirectory(baseLocationUri);
     } catch (Exception e) {
       final Throwable cause = e.getCause();
-      if (cause != null) {
-        assumeFalse(
-            "This test uses a GCS mock library that is incompatible with the current default locale",
-            e instanceof StorageException
-                && cause.getMessage().contains("Invalid date/time format")
-                && cause instanceof NumberFormatException);
-      }
+      assumeFalse(
+          "This test uses a GCS mock library that is incompatible with the current default locale",
+          cause != null
+              && e instanceof StorageException
+              && cause.getMessage().contains("Invalid date/time format")
+              && cause instanceof NumberFormatException);
+      // Not the known locale incompatibility - a genuine failure, so don't swallow it.
+      throw new RuntimeException(e);
     }
   }
 }