diff --git a/src/buildstream/sandbox/_sandboxreapi.py b/src/buildstream/sandbox/_sandboxreapi.py
index 9b95f63fa..be210e450 100644
--- a/src/buildstream/sandbox/_sandboxreapi.py
+++ b/src/buildstream/sandbox/_sandboxreapi.py
@@ -184,8 +184,13 @@ def _fetch_action_result_outputs(self, casremote, action_result):
 
             raise SandboxError("Output directory structure had no digest attached.")
 
-        # Fetch stdout and stderr blobs
-        cascache.fetch_blobs(casremote, [action_result.stdout_digest, action_result.stderr_digest])
+        # Fetch stdout and stderr blobs, if they exist
+        blobs = []
+        for digest in [action_result.stdout_digest, action_result.stderr_digest]:
+            if digest.hash:
+                blobs.append(digest)
+        if blobs:
+            cascache.fetch_blobs(casremote, blobs)
 
     def _process_job_output(self, working_directory, output_directories, output_files, *, failure):
         # Reads the remote execution server response to an execution request.