diff --git a/include/frozen/bits/basic_types.h b/include/frozen/bits/basic_types.h
index 6483072..2d77604 100644
--- a/include/frozen/bits/basic_types.h
+++ b/include/frozen/bits/basic_types.h
@@ -99,7 +99,7 @@ class carray {
 
   static constexpr void check_initializer(std::initializer_list<T> init) {
     (void)init;
-    constexpr_assert(init.size() >= N, "Cannot initialize a carray with an smaller initializer list");
+    constexpr_assert(init.size() == N, "Cannot initialize a carray with an initializer list of different size.");
   }
 
 public:
diff --git a/include/frozen/bits/pmh.h b/include/frozen/bits/pmh.h
index 1bb4021..4fb2bef 100644
--- a/include/frozen/bits/pmh.h
+++ b/include/frozen/bits/pmh.h
@@ -184,15 +184,23 @@ struct pmh_tables : private Hasher {
 };
 
 // Make pmh tables for given items, hash function, prg, etc.
-template <std::size_t M, class Item, std::size_t N, class Hash, class Key, class PRG>
+template <std::size_t M, class Item, std::size_t N, class Hash, class Key, class KeyEqual, class PRG>
 pmh_tables<M, Hash> constexpr make_pmh_tables(const carray<Item, N> &
                                                                items,
                                                            Hash const &hash,
+                                                           KeyEqual const &equal,
                                                            Key const &key,
                                                            PRG prg) {
   // Step 1: Place all of the keys into buckets
   auto step_one = make_pmh_buckets<M>(items, hash, key, prg);
 
+#ifndef NDEBUG
+  // Step 1.5: Detect redundant keys.
+  for(auto const& bucket : step_one.buckets)
+    for(std::size_t i = 1; i < bucket.size(); ++i)
+      constexpr_assert(!equal(key(items[0]), key(items[i])), "unique keys");
+#endif
+
   // Step 2: Sort the buckets to process the ones with the most items first.
   auto buckets = step_one.get_sorted_buckets();
 
diff --git a/include/frozen/unordered_map.h b/include/frozen/unordered_map.h
index 7c0ffb3..2405385 100644
--- a/include/frozen/unordered_map.h
+++ b/include/frozen/unordered_map.h
@@ -83,7 +83,7 @@ class unordered_map : private KeyEqual {
       , items_{items}
       , tables_{
             bits::make_pmh_tables<storage_size>(
-                items_, hash, bits::GetKey{}, default_prg_t{})} {}
+                items_, hash, equal, bits::GetKey{}, default_prg_t{})} {}
   explicit constexpr unordered_map(container_type items)
       : unordered_map{items, Hash{}, KeyEqual{}} {
       }
diff --git a/include/frozen/unordered_set.h b/include/frozen/unordered_set.h
index 0c53b15..482034b 100644
--- a/include/frozen/unordered_set.h
+++ b/include/frozen/unordered_set.h
@@ -77,7 +77,7 @@ class unordered_set : private KeyEqual {
       : KeyEqual{equal}
       , keys_{keys}
       , tables_{bits::make_pmh_tables<storage_size>(
-            keys_, hash, bits::Get{}, default_prg_t{})} {}
+            keys_, hash, equal, bits::Get{}, default_prg_t{})} {}
   explicit constexpr unordered_set(container_type keys)
       : unordered_set{keys, Hash{}, KeyEqual{}} {}