diff --git a/test/runner/lib/util.py b/test/runner/lib/util.py
index 3ca4e04536..a8f88a8e76 100644
--- a/test/runner/lib/util.py
+++ b/test/runner/lib/util.py
@@ -381,7 +381,10 @@ def raw_command(cmd, capture=False, env=None, data=None, cwd=None, explain=False
 
     if communicate:
         encoding = 'utf-8'
-        data_bytes = data.encode(encoding, 'surrogateescape') if data else None
+        if data is None or isinstance(data, bytes):
+            data_bytes = data
+        else:
+            data_bytes = data.encode(encoding, 'surrogateescape')
         stdout_bytes, stderr_bytes = process.communicate(data_bytes)
         stdout_text = stdout_bytes.decode(encoding, str_errors) if stdout_bytes else u''
         stderr_text = stderr_bytes.decode(encoding, str_errors) if stderr_bytes else u''