diff --git a/SkipList.cpp b/SkipList.cpp
index 5dee515..e70a60c 100644
--- a/SkipList.cpp
+++ b/SkipList.cpp
@@ -99,8 +99,16 @@ force_inline bool getCharacter(const KeyInfo &ki, int character,
 }
 
 bool operator<(const KeyInfo &lhs, const KeyInfo &rhs) {
+  // An empty key is a prefix of every key, so it sorts before any non-empty
+  // key. Handle empty keys up front: an empty std::span may have
+  // data() == nullptr, which must never be passed to memcmp.
+  if (lhs.key.size() == 0 || rhs.key.size() == 0) {
+    if (lhs.key.size() != rhs.key.size())
+      return lhs.key.size() < rhs.key.size();
+    return extra_ordering(lhs) < extra_ordering(rhs);
+  }
   int i = std::min(lhs.key.size(), rhs.key.size());
-  int c = i > 0 ? memcmp(lhs.key.data(), rhs.key.data(), i) : 0;
+  int c = memcmp(lhs.key.data(), rhs.key.data(), i);
   if (c != 0)
     return c < 0;