diff --git a/src/vmemsave.c b/src/vmemsave.c
index 9dc9aaf..7482446 100644
--- a/src/vmemsave.c
+++ b/src/vmemsave.c
@@ -205,7 +205,7 @@ LispPTR vmem_save0(LispPTR *args)
 /************************************************************************/
 
 int twowords(const void *i, const void *j) /* the difference between two  DLwords. */
-{ return (*(DLword *)i - *(DLword *)j); }
+{ return (*(const DLword *)i - *(const DLword *)j); }
 
 #define FPTOVP_ENTRY (FPTOVP_OFFSET >> 8)