diff --git a/src/zmalloc.c b/src/zmalloc.c
index 087078996f..95096efa31 100644
--- a/src/zmalloc.c
+++ b/src/zmalloc.c
@@ -364,11 +364,11 @@ size_t zmalloc_usable_size(void *ptr) {
 /* Frees the memory buffer pointed by ptr and updates statistics. When using
  * jemalloc it uses the fast track by specifying the buffer size.
  *
- * ptr must point to the start of the buffer. On systems where we have the
- * additional header with size, the caller must do the necessary adjustments to
- * ptr. ptr must not be NULL. The caller is responsible to provide the real
- * allocaction size. */
+ * ptr must point to the start of the buffer. It must have been returned by a
+ * previous call to the system allocator. ptr must not be NULL. The caller is
+ * responsible to provide the real allocaction size. */
 static inline void zfree_internal(void *ptr, size_t size) {
+    assert(ptr != NULL);
     update_zmalloc_stat_free(size);
 
 #ifdef USE_JEMALLOC