diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/backup/BackupManager.java b/service/src/main/java/org/whispersystems/textsecuregcm/backup/BackupManager.java
index f8539db45..e03f8278c 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/backup/BackupManager.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/backup/BackupManager.java
@@ -8,6 +8,7 @@ package org.whispersystems.textsecuregcm.backup;
 import com.google.common.annotations.VisibleForTesting;
 import io.grpc.Status;
 import io.micrometer.core.instrument.Metrics;
+import java.io.IOException;
 import java.net.URI;
 import java.time.Clock;
 import java.time.Duration;
@@ -248,12 +249,17 @@ public class BackupManager {
           .asRuntimeException();
     }
 
+    final URI sourceUri;
+    try {
+      sourceUri = attachmentReadUri(sourceCdn, sourceKey);
+    } catch (IOException e) {
+      return CompletableFuture.failedFuture(e);
+    }
+
     final MessageBackupUploadDescriptor dst = cdn3BackupCredentialGenerator.generateUpload(
         cdnMediaPath(backupUser, destinationMediaId));
 
     final int destinationLength = encryptionParameters.outputSize(sourceLength);
-
-    final URI sourceUri = attachmentReadUri(sourceCdn, sourceKey);
     return this.backupsDb
         // Write the ddb updates before actually updating backing storage
         .trackMedia(backupUser, 1, destinationLength)
@@ -282,10 +288,10 @@ public class BackupManager {
    * @param key the attachment key
    * @return A {@link URI} where the attachment can be retrieved
    */
-  private URI attachmentReadUri(final int cdn, final String key) {
+  private URI attachmentReadUri(final int cdn, final String key) throws IOException {
     final String baseUri = attachmentCdnBaseUris.get(cdn);
     if (baseUri == null) {
-      throw Status.INVALID_ARGUMENT.withDescription("Unknown cdn " + cdn).asRuntimeException();
+      throw new SourceObjectNotFoundException("Unknown attachment cdn " + cdn);
     }
     return URI.create("%s/%s".formatted(baseUri, key));
   }
diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/backup/SourceObjectNotFoundException.java b/service/src/main/java/org/whispersystems/textsecuregcm/backup/SourceObjectNotFoundException.java
index 5d2b20f3b..078a1d13e 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/backup/SourceObjectNotFoundException.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/backup/SourceObjectNotFoundException.java
@@ -8,4 +8,10 @@ package org.whispersystems.textsecuregcm.backup;
 import java.io.IOException;
 
 public class SourceObjectNotFoundException extends IOException {
+  public SourceObjectNotFoundException() {
+    super();
+  }
+  public SourceObjectNotFoundException(String message) {
+    super(message);
+  }
 }
diff --git a/service/src/test/java/org/whispersystems/textsecuregcm/backup/BackupManagerTest.java b/service/src/test/java/org/whispersystems/textsecuregcm/backup/BackupManagerTest.java
index 4a5540925..8fa04e9eb 100644
--- a/service/src/test/java/org/whispersystems/textsecuregcm/backup/BackupManagerTest.java
+++ b/service/src/test/java/org/whispersystems/textsecuregcm/backup/BackupManagerTest.java
@@ -39,7 +39,6 @@ import java.util.UUID;
 import java.util.concurrent.CompletableFuture;
 import javax.annotation.Nullable;
 import org.apache.commons.lang3.StringUtils;
-import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.RegisterExtension;
@@ -308,6 +307,17 @@ public class BackupManagerTest {
     assertThat(AttributeValues.getLong(backup, BackupsDb.ATTR_MEDIA_COUNT, -1L)).isEqualTo(0L);
   }
 
+  @Test
+  public void unknownSourceCdn() {
+    final AuthenticatedBackupUser backupUser = backupUser(TestRandomUtil.nextBytes(16), BackupTier.MEDIA);
+    CompletableFutureTestUtil.assertFailsWithCause(SourceObjectNotFoundException.class,
+        backupManager.copyToBackup(
+            backupUser,
+            0, "abc", 100,
+            mock(MediaEncryptionParameters.class),
+            "def".getBytes(StandardCharsets.UTF_8)));
+  }
+
   @Test
   public void quotaEnforcementNoRecalculation() {
     final AuthenticatedBackupUser backupUser = backupUser(TestRandomUtil.nextBytes(16), BackupTier.MEDIA);