diff --git a/tools/libs/foreignmemory/private.h b/tools/libs/foreignmemory/private.h
index 8f1bf081ed..e7b2cfa28a 100644
--- a/tools/libs/foreignmemory/private.h
+++ b/tools/libs/foreignmemory/private.h
@@ -32,7 +32,7 @@ int osdep_xenforeignmemory_close(xenforeignmemory_handle *fmem);
 void *osdep_xenforeignmemory_map(xenforeignmemory_handle *fmem,
                                  uint32_t dom, void *addr,
                                  int prot, int flags, size_t num,
-                                 const xen_pfn_t arr[num], int err[num]);
+                                 const xen_pfn_t arr[/*num*/], int err[/*num*/]);
 int osdep_xenforeignmemory_unmap(xenforeignmemory_handle *fmem,
                                  void *addr, size_t num);