diff --git a/src/segment-cache.c b/src/segment-cache.c
index 34c8b029..436ce2bf 100644
--- a/src/segment-cache.c
+++ b/src/segment-cache.c
@@ -285,8 +285,9 @@ void _mi_segment_map_freed_at(const mi_segment_t* segment) {
 
 // Determine the segment belonging to a pointer or NULL if it is not in a valid segment.
 static mi_segment_t* _mi_segment_of(const void* p) {
+  if (p == NULL) return NULL;
   mi_segment_t* segment = _mi_ptr_segment(p);
-  if (segment == NULL) return NULL; 
+  mi_assert_internal(segment != NULL);
   size_t bitidx;
   size_t index = mi_segment_map_index_of(segment, &bitidx);
   // fast path: for any pointer to valid small/medium/large object or first MI_SEGMENT_SIZE in huge