diff --git a/src/libutil/git.cc b/src/libutil/git.cc
index 0b6e35222c2..a60589baa17 100644
--- a/src/libutil/git.cc
+++ b/src/libutil/git.cc
@@ -251,6 +251,7 @@ void dumpTree(const Tree & entries, Sink & sink,
     for (auto & [name, entry] : entries) {
         auto name2 = name;
         if (entry.mode == Mode::Directory) {
+            assert(!name2.empty());
             assert(name2.back() == '/');
             name2.pop_back();
         }