diff --git a/glib/gslice.c b/glib/gslice.c
index 11d8c3fb2..63a048458 100644
--- a/glib/gslice.c
+++ b/glib/gslice.c
@@ -331,7 +331,7 @@ void
 g_slice_free1 (gsize    mem_size,
                gpointer mem_block)
 {
-  if (G_UNLIKELY (g_mem_gc_friendly))
+  if (G_UNLIKELY (g_mem_gc_friendly && mem_block))
     memset (mem_block, 0, mem_size);
   g_free_sized (mem_block, mem_size);
   TRACE (GLIB_SLICE_FREE((void*)mem_block, mem_size));