diff --git a/utils/lit/TestRunner.py b/utils/lit/TestRunner.py
index bee1167f6af..7abc60a1f00 100644
--- a/utils/lit/TestRunner.py
+++ b/utils/lit/TestRunner.py
@@ -112,6 +112,9 @@ def executeShCmd(cmd, cfg, cwd, results):
                         r[2] = tempfile.TemporaryFile(mode=r[1])
                     else:
                         r[2] = open(r[0], r[1])
+                    # Workaround a Win32 and/or subprocess bug when appending.
+                    if r[1] == 'a':
+                        r[2].seek(0, os.SEEK_END)
                 result = r[2]
             final_redirects.append(result)