Index: pack.c
===================================================================
--- pack.c	(revision 46990)
+++ pack.c	(working copy)
@@ -946,8 +946,8 @@
 static void
 encodes(VALUE str, const char *s, long len, int type, int tail_lf)
 {
-    enum {buff_size = 4096, encoded_unit = 4};
-    char buff[buff_size + 1];	/* +1 for tail_lf */
+    enum {buff_size = 256, encoded_unit = 4};
+    char buff[buff_size];
     long i = 0;
     const char *trans = type == 'u' ? uu_table : b64_table;
     char padding;
@@ -968,7 +968,7 @@
             s += 3;
             len -= 3;
         }
-        if (buff_size-i < encoded_unit) {
+        if ( ( buff_size-i < encoded_unit ) || len < 3 ) {
             rb_str_buf_cat(str, buff, i);
             i = 0;
         }