diff --git a/src/snmalloc/backend_helpers/pagemap.h b/src/snmalloc/backend_helpers/pagemap.h
index dd0b313..531196c 100644
--- a/src/snmalloc/backend_helpers/pagemap.h
+++ b/src/snmalloc/backend_helpers/pagemap.h
@@ -58,6 +58,8 @@ namespace snmalloc
      */
     void register_range(address_t p, size_t length)
     {
+      SNMALLOC_ASSERT(is_initialised());
+
       // Calculate range in pagemap that is associated to this space.
       auto first = &body[p >> SHIFT];
       auto last = &body[(p + length + bits::one_at_bit(SHIFT) - 1) >> SHIFT];
@@ -96,6 +98,8 @@ namespace snmalloc
     template<bool has_bounds_ = has_bounds>
     std::enable_if_t<!has_bounds_> init(T* address)
     {
+      SNMALLOC_ASSERT(!is_initialised());
+
       static_assert(
         has_bounds_ == has_bounds, "Don't set SFINAE template parameter!");
       body = address;
@@ -111,6 +115,8 @@ namespace snmalloc
     std::enable_if_t<has_bounds_, std::pair<void*, size_t>>
     init(void* b, size_t s)
     {
+      SNMALLOC_ASSERT(!is_initialised());
+
       static_assert(
         has_bounds_ == has_bounds, "Don't set SFINAE template parameter!");
 #ifdef SNMALLOC_TRACING
@@ -167,6 +173,8 @@ namespace snmalloc
     template<bool has_bounds_ = has_bounds>
     std::enable_if_t<!has_bounds_> init()
     {
+      SNMALLOC_ASSERT(!is_initialised());
+
       static_assert(
         has_bounds_ == has_bounds, "Don't set SFINAE template parameter!");
       static constexpr size_t REQUIRED_SIZE = required_size();
@@ -222,6 +230,8 @@ namespace snmalloc
     template<bool has_bounds_ = has_bounds>
     std::enable_if_t<has_bounds_, std::pair<address_t, size_t>> get_bounds()
     {
+      SNMALLOC_ASSERT(is_initialised());
+
       static_assert(
         has_bounds_ == has_bounds, "Don't set SFINAE template parameter!");
 
@@ -233,6 +243,8 @@ namespace snmalloc
      */
     [[nodiscard]] constexpr size_t num_entries() const
     {
+      SNMALLOC_ASSERT(is_initialised());
+
       if constexpr (has_bounds)
       {
         return size >> GRANULARITY_BITS;
@@ -258,6 +270,8 @@ namespace snmalloc
           return const_cast<T&>(default_value);
       }
 
+      SNMALLOC_ASSERT(is_initialised() || p == 0);
+
       if constexpr (has_bounds)
       {
         if (p - base > size)
@@ -318,6 +332,7 @@ namespace snmalloc
      */
     [[nodiscard]] address_t get_address(const T& t) const
     {
+      SNMALLOC_ASSERT(is_initialised());
       address_t entry_offset = address_cast(&t) - address_cast(body);
       address_t entry_index = entry_offset / sizeof(T);
       SNMALLOC_ASSERT(
@@ -327,6 +342,7 @@ namespace snmalloc
 
     void set(address_t p, const T& t)
     {
+      SNMALLOC_ASSERT(is_initialised());
 #ifdef SNMALLOC_TRACING
       message<1024>("Pagemap.Set {}", p);
 #endif
diff --git a/src/snmalloc/override/malloc.cc b/src/snmalloc/override/malloc.cc
index 5830f94..14e770b 100644
--- a/src/snmalloc/override/malloc.cc
+++ b/src/snmalloc/override/malloc.cc
@@ -154,6 +154,8 @@ extern "C"
     }
 
     sz = bits::min(sz, a.alloc_size(*ptr));
+
+    SNMALLOC_ASSUME(*ptr != nullptr || sz == 0);
     // Guard memcpy as GCC is assuming not nullptr for ptr after the memcpy
     // otherwise.
     if (sz != 0)