Index: array.c
===================================================================
--- array.c	(revision 33907)
+++ array.c	(working copy)
@@ -1730,8 +1730,8 @@
     VALUE val, tmp, result;
 
     if (RARRAY_LEN(ary) == 0) return rb_usascii_str_new(0, 0);
-    if (OBJ_TAINTED(ary) || OBJ_TAINTED(sep)) taint = TRUE;
-    if (OBJ_UNTRUSTED(ary) || OBJ_UNTRUSTED(sep)) untrust = TRUE;
+    if (OBJ_TAINTED(ary) || OBJ_TAINTED(sep)&&RARRAY_LEN(ary)>1) taint = TRUE;
+    if (OBJ_UNTRUSTED(ary) || OBJ_UNTRUSTED(sep)&&RARRAY_LEN(ary)>1) untrust = TRUE;
 
     if (!NIL_P(sep)) {
 	StringValue(sep);