diff --git a/libavutil/mem.c b/libavutil/mem.c
index 8cad089a7d..18fe28b7ce 100644
--- a/libavutil/mem.c
+++ b/libavutil/mem.c
@@ -61,7 +61,7 @@ void  free(void *ptr);
    memory allocator. You do not need to suppress this file because the
    linker will do it automatically. */
 
-void *av_malloc(unsigned int size)
+void *av_malloc(FF_INTERNAL_MEM_TYPE size)
 {
     void *ptr = NULL;
 #if CONFIG_MEMALIGN_HACK
@@ -116,7 +116,7 @@ void *av_malloc(unsigned int size)
     return ptr;
 }
 
-void *av_realloc(void *ptr, unsigned int size)
+void *av_realloc(void *ptr, FF_INTERNAL_MEM_TYPE size)
 {
 #if CONFIG_MEMALIGN_HACK
     int diff;
@@ -154,7 +154,7 @@ void av_freep(void *arg)
     *ptr = NULL;
 }
 
-void *av_mallocz(unsigned int size)
+void *av_mallocz(FF_INTERNAL_MEM_TYPE size)
 {
     void *ptr = av_malloc(size);
     if (ptr)
diff --git a/libavutil/mem.h b/libavutil/mem.h
index 1ec8eb8601..458d3aced1 100644
--- a/libavutil/mem.h
+++ b/libavutil/mem.h
@@ -62,6 +62,12 @@
     #define av_alloc_size(n)
 #endif
 
+#if LIBAVUTIL_VERSION_MAJOR < 51
+#   define FF_INTERNAL_MEM_TYPE unsigned int
+#else
+#   define FF_INTERNAL_MEM_TYPE size_t
+#endif
+
 /**
  * Allocate a block of size bytes with alignment suitable for all
  * memory accesses (including vectors if available on the CPU).
@@ -70,7 +76,7 @@
  * be allocated.
  * @see av_mallocz()
  */
-void *av_malloc(unsigned int size) av_malloc_attrib av_alloc_size(1);
+void *av_malloc(FF_INTERNAL_MEM_TYPE size) av_malloc_attrib av_alloc_size(1);
 
 /**
  * Allocate or reallocate a block of memory.
@@ -84,7 +90,7 @@ void *av_malloc(unsigned int size) av_malloc_attrib av_alloc_size(1);
  * cannot be reallocated or the function is used to free the memory block.
  * @see av_fast_realloc()
  */
-void *av_realloc(void *ptr, unsigned int size) av_alloc_size(2);
+void *av_realloc(void *ptr, FF_INTERNAL_MEM_TYPE size) av_alloc_size(2);
 
 /**
  * Free a memory block which has been allocated with av_malloc(z)() or
@@ -104,7 +110,7 @@ void av_free(void *ptr);
  * @return Pointer to the allocated block, NULL if it cannot be allocated.
  * @see av_malloc()
  */
-void *av_mallocz(unsigned int size) av_malloc_attrib av_alloc_size(1);
+void *av_mallocz(FF_INTERNAL_MEM_TYPE size) av_malloc_attrib av_alloc_size(1);
 
 /**
  * Duplicate the string s.