diff --git a/os/lib/heapmem.c b/os/lib/heapmem.c
index 4e3d372805f..99a879f32b0 100644
--- a/os/lib/heapmem.c
+++ b/os/lib/heapmem.c
@@ -172,7 +172,8 @@ static size_t heap_usage;
 static chunk_t *first_chunk = (chunk_t *)heap_base;
 static chunk_t *free_list;
 
-#define IN_HEAP(ptr) ((char *)(ptr) >= (char *)heap_base) && \
+#define IN_HEAP(ptr) ((ptr) != NULL && \
+                     (char *)(ptr) >= (char *)heap_base) && \
                      ((char *)(ptr) < (char *)heap_base + heap_usage)
 
 /* extend_space: Increases the current footprint used in the heap, and
diff --git a/os/lib/heapmem.h b/os/lib/heapmem.h
index 874a5c36bdc..9e83014b60b 100644
--- a/os/lib/heapmem.h
+++ b/os/lib/heapmem.h
@@ -38,9 +38,7 @@
  * The heapmem module is a dynamic heap memory allocator similar to
  * malloc() in standard C. The heap memory is managed in a block of
  * static memory, whose size is determined at compile-time by setting
- * HEAPMEM_CONF_ARENA_SIZE parameter. By default, the heap memory is
- * only 1 bytes, which entails that this parameter must be set
- * explicitly in order to be possible to use this module.
+ * the HEAPMEM_CONF_ARENA_SIZE parameter.
  *
  * Each allocated memory object is referred to as a "chunk". The
  * allocator manages free chunks in a double-linked list. While this
@@ -60,7 +58,7 @@
  *       standard C function calloc().
  *
  * \note Dynamic memory should be used carefully on
- *       memory-constrained, embedded systems, because fragmentation
+ *       memory-constrained embedded systems, because fragmentation
  *       may be induced through various allocation/deallocation
  *       patterns, and no guarantees are given regarding the
  *       availability of memory.
@@ -105,7 +103,8 @@ typedef uint8_t heapmem_zone_t;
  * \param name A pointer to a chunk that has been allocated using
  *             heapmem_alloc() or heapmem_realloc().
  * \param zone_size The number of bytes to reserve for the zone.
- * \return     A zone ID if the allocation succeeds, or HEAPMEM_ZONE_INVALID if it fails.
+ * \return     A zone ID if the allocation succeeds, or
+ *             HEAPMEM_ZONE_INVALID if it fails.
  */
 heapmem_zone_t heapmem_zone_register(const char *name, size_t zone_size);
 /*****************************************************************************/
@@ -177,7 +176,7 @@ void *heapmem_realloc(void *ptr, size_t size);
  * \return     A boolean indicating whether the memory could be deallocated.
  *
  * \note If ptr is NULL, this function will return immediately without
- *       without performing any action.
+ *       performing any action.
  *
  * \sa         heapmem_alloc
  * \sa         heapmem_realloc