diff --git a/src/PackCommand.cc b/src/PackCommand.cc
index 4059cf1b6cf..378e4df1f11 100644
--- a/src/PackCommand.cc
+++ b/src/PackCommand.cc
@@ -754,6 +754,11 @@ int PackCommand::run(vector<string>& args) {
     trace_dirs.push_back(trace_dir);
   }
 
+  // If no trace dirs were supplied, default to packing "", i.e. the latest trace.
+  if (trace_dirs.empty()) {
+    trace_dirs.push_back("");
+  }
+
   return pack(trace_dirs, flags);
 }
 
diff --git a/src/test/pack.run b/src/test/pack.run
index 3eaae5a0050..4c196d7c67b 100644
--- a/src/test/pack.run
+++ b/src/test/pack.run
@@ -12,6 +12,11 @@ pack || failed "'rr pack' failed"
 if [[ `ls latest-trace/*_pack$bitness-* | wc -l` != "1" ]]; then
     failed "Failed to coalesce all instances of 'pack'"
 fi
+
+if [[ `ls latest-trace/mmap_pack_* | wc -l` == "0" ]]; then
+    failed "Packing produced no files."
+fi
+
 # If there are no copies of _mapped_file it was probably copied
 # into the trace data, for one of various reasons, so don't fail
 # the test in that case.