diff --git a/src/h/external.h b/src/h/external.h
index e87aeac5a..692409f04 100644
--- a/src/h/external.h
+++ b/src/h/external.h
@@ -27,8 +27,8 @@ extern cl_object make_cons(cl_object a, cl_object d);
 extern void cl_dealloc(void *p, cl_index s);
 #ifdef GBC_BOEHM
 extern cl_object cl_gc(cl_object area);
-extern void *GC_malloc(cl_index size);
-extern void *GC_malloc_atomic_ignore_off_page(cl_index size);
+extern void *GC_malloc(size_t size);
+extern void *GC_malloc_atomic_ignore_off_page(size_t size);
 extern void GC_free(void *);
 #define cl_alloc GC_malloc
 #define cl_alloc_atomic GC_malloc_atomic_ignore_off_page