diff -pruN ruby-1.8.6-p230.orig/string.c ruby-1.8.6-p230/string.c
--- ruby-1.8.6-p230.orig/string.c	2008-06-20 18:24:53.000000000 +0900
+++ ruby-1.8.6-p230/string.c	2008-06-25 16:10:17.000000000 +0900
@@ -693,9 +693,13 @@ str_buf_cat(str, ptr, len)
     const char *ptr;
     long len;
 {
-    long capa, total;
+    long capa, total, off = -1;
 
     rb_str_modify(str);
+    if (ptr >= RSTRING(str)->ptr && ptr <= RSTRING(str)->ptr + RSTRING(str)->len) {
+	off = ptr - RSTRING(str)->ptr;
+    }
+    if (len == 0) return 0;
     if (FL_TEST(str, STR_ASSOC)) {
 	FL_UNSET(str, STR_ASSOC);
 	capa = RSTRING(str)->aux.capa = RSTRING(str)->len;
@@ -717,6 +721,9 @@ str_buf_cat(str, ptr, len)
 	}
 	RESIZE_CAPA(str, capa);
     }
+    if (off != -1) {
+	ptr = RSTRING(str)->ptr + off;
+    }
     memcpy(RSTRING(str)->ptr + RSTRING(str)->len, ptr, len);
     RSTRING(str)->len = total;
     RSTRING(str)->ptr[total] = '\0'; /* sentinel */