diff --git a/tools/worker/swift_runner.cc b/tools/worker/swift_runner.cc
index cd6279913..eca0f2993 100644
--- a/tools/worker/swift_runner.cc
+++ b/tools/worker/swift_runner.cc
@@ -203,7 +203,11 @@ bool SwiftRunner::ProcessArgument(
     // Apply any other text substitutions needed in the argument (i.e., for
     // Apple toolchains).
     auto new_arg = arg;
-    changed = MakeSubstitutions(&new_arg, bazel_placeholder_substitutions_);
+    // Bazel doesn't quote arguments in multi-line params files, so we need to
+    // ensure that our defensive quoting kicks in if an argument contains a
+    // space, even if no other changes would have been made.
+    changed = MakeSubstitutions(&new_arg, bazel_placeholder_substitutions_) ||
+              new_arg.find_first_of(' ') != std::string::npos;
     consumer(new_arg);
   }