diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -842,6 +842,7 @@ def main():
     os.environ["EMAIL"] = "Foo Bar <foo.bar@example.com>"
     os.environ['CDPATH'] = ''
     os.environ['COLUMNS'] = '80'
+    os.environ['GREP_OPTIONS'] = ''
     os.environ['http_proxy'] = ''
 
     global TESTDIR, HGTMP, INST, BINDIR, PYTHONDIR, COVERAGE_FILE