diff --git a/src/ops.c b/src/ops.c
index 72ffd56..28e9757 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -3813,8 +3813,12 @@
 
 	bom_count = bomb_size();
 	if (dict == NULL && bom_count > 0)
-	    vim_snprintf((char *)IObuff + STRLEN(IObuff), IOSIZE,
+	{
+	    size_t len = STRLEN(IObuff);
+
+	    vim_snprintf((char *)IObuff + len, IOSIZE - len,
 				 _("(+%lld for BOM)"), (long_long_T)bom_count);
+	}
 	if (dict == NULL)
 	{
 	    // Don't shorten this message, the user asked for it.