diff --git a/cxxmph/mph_index.cc b/cxxmph/mph_index.cc
index 8b6baec..5a6426e 100644
--- a/cxxmph/mph_index.cc
+++ b/cxxmph/mph_index.cc
@@ -180,20 +180,26 @@ void MPHIndex::Ranking() {
 }
 
 uint32_t MPHIndex::Rank(uint32_t vertex) const {
+  if (!g_.size()) return 0;
   uint32_t index = vertex >> b_;
   uint32_t base_rank = ranktable_[index];
   uint32_t beg_idx_v = index << b_;
   uint32_t beg_idx_b = beg_idx_v >> 2;
   uint32_t end_idx_b = vertex >> 2;
-  while (beg_idx_b < end_idx_b) base_rank += kBdzLookupIndex[g_.data()[beg_idx_b++]];
+  while (beg_idx_b < end_idx_b) {
+     assert(g_.data().size() > beg_idx_b);
+     base_rank += kBdzLookupIndex[g_.data()[beg_idx_b++]];
+  }
   beg_idx_v = beg_idx_b << 2;
-  // cerr << "beg_idx_v: " << beg_idx_v << endl;
-  // cerr << "base rank: " << base_rank << endl;
-  // cerr << "G: ";
-  // for (unsigned int i = 0; i < n_; ++i) {
-  //  cerr << static_cast<uint32_t>(g_[i]) << " ";
-  // }
-  // cerr << endl;
+  /*
+  cerr << "beg_idx_v: " << beg_idx_v << endl;
+  cerr << "base rank: " << base_rank << endl;
+  cerr << "G: ";
+  for (unsigned int i = 0; i < n_; ++i) {
+   cerr << static_cast<uint32_t>(g_[i]) << " ";
+  }
+  cerr << endl;
+  */
   while (beg_idx_v < vertex) {
     if (g_[beg_idx_v] != kUnassigned) ++base_rank;
     ++beg_idx_v;
diff --git a/cxxmph/mph_index.h b/cxxmph/mph_index.h
index 6982534..11d9bea 100644
--- a/cxxmph/mph_index.h
+++ b/cxxmph/mph_index.h
@@ -65,6 +65,7 @@ class MPHIndex {
   uint32_t perfect_hash_size() const { return n_; }
   template <class SeededHashFcn, class Key>  // must agree with Reset
   uint32_t perfect_hash(const Key& x) const;
+  uint32_t minimal_perfect_hash_size() const { return size(); }
   template <class SeededHashFcn, class Key>  // must agree with Reset
   uint32_t minimal_perfect_hash(const Key& x) const;
 
diff --git a/cxxmph/mph_map.h b/cxxmph/mph_map.h
index 60a52a6..ceb3109 100644
--- a/cxxmph/mph_map.h
+++ b/cxxmph/mph_map.h
@@ -172,6 +172,7 @@ MPH_MAP_METHOD_DECL(void_type, pack)() {
   new_present.reserve(new_present.size() * 2);
   for (iterator it = begin(), it_end = end(); it != it_end; ++it) {
     size_type id = index_.perfect_hash(it->first);
+    assert(id < index_.perfect_hash_size());
     assert(id < new_values.size());
     new_values[id] = *it;
     new_present[id] = true;