diff --git a/tests/littlecheck.py b/tests/littlecheck.py
index d07c134f9..e3d843826 100755
--- a/tests/littlecheck.py
+++ b/tests/littlecheck.py
@@ -367,7 +367,10 @@ def perform_substitution(input_str, subs):
         text = m.group(1)
         for key, replacement in subs_ordered:
             if text.startswith(key):
-                return replacement + text[len(key) :]
+                # shell-quote the replacement, so it's usable in #RUN lines.
+                # We could loosen this and only do it for #RUN/#REQUIRES,
+                # but so far we don't need it anywhere.
+                return shlex.quote(replacement + text[len(key) :])
         # No substitution found, so we default to running it as-is,
         # which will end up running it via $PATH.
         return text
@@ -475,7 +478,7 @@ class TestRun(object):
             """Decode a string and split it by newlines only,
             retaining the newlines.
             """
-            return [s + "\n" for s in s.decode("utf-8").split("\n")]
+            return [s + "\n" for s in s.decode("utf-8", errors="backslashreplace").split("\n")]
 
         if self.config.verbose:
             print(self.subbed_command)