diff --git a/bsd-user/mmap.c b/bsd-user/mmap.c
index 66233abc11..e916a6544e 100644
--- a/bsd-user/mmap.c
+++ b/bsd-user/mmap.c
@@ -127,6 +127,8 @@ void *qemu_realloc(void *ptr, size_t size)
     size_t old_size, copy;
     void *new_ptr;
 
+    if (!ptr)
+        return qemu_malloc(size);
     old_size = *(size_t *)((char *)ptr - 16);
     copy = old_size < size ? old_size : size;
     new_ptr = qemu_malloc(size);
diff --git a/linux-user/mmap.c b/linux-user/mmap.c
index d0fc3e3d1a..888b491403 100644
--- a/linux-user/mmap.c
+++ b/linux-user/mmap.c
@@ -128,6 +128,8 @@ void *qemu_realloc(void *ptr, size_t size)
     size_t old_size, copy;
     void *new_ptr;
 
+    if (!ptr)
+        return qemu_malloc(size);
     old_size = *(size_t *)((char *)ptr - 16);
     copy = old_size < size ? old_size : size;
     new_ptr = qemu_malloc(size);