diff --git a/ChangeLog b/ChangeLog
index be3aca7c07..841643ce55 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Jan 11 12:47:58 2010  Akinori MUSHA  <knu@iDaemons.org>
+
+	* hash.c (ruby_setenv): ENV.[]= should raise an error if setenv(3)
+	  or putenv(3) fails.  [ruby-dev:40023]
+
 Sun Jan 10 17:25:24 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/webrick/accesslog.rb : Escape needed.
diff --git a/NEWS b/NEWS
index be0807b747..c152cf58a9 100644
--- a/NEWS
+++ b/NEWS
@@ -45,6 +45,7 @@ with all sufficient information, see the ChangeLog file.
 
   * ENV
     * Uses locale's encoding
+    * ENV.[]= raises Errno::{EINVAL,ENOMEM} etc. on failure.
 
   * Float
     * new constants:
diff --git a/hash.c b/hash.c
index 321f7a7375..ca691f4dd4 100644
--- a/hash.c
+++ b/hash.c
@@ -2064,10 +2064,13 @@ ruby_setenv(const char *name, const char *value)
 #elif defined(HAVE_SETENV) && defined(HAVE_UNSETENV)
 #undef setenv
 #undef unsetenv
-    if (value)
-	setenv(name,value,1);
-    else
-	unsetenv(name);
+    if (value) {
+	if (setenv(name, value, 1))
+	    rb_sys_fail("setenv");
+    } else {
+	if (unsetenv(name))
+	    rb_sys_fail("unsetenv");
+    }
 #elif defined __sun__
     size_t len = strlen(name);
     char **env_ptr, *str;
@@ -2081,7 +2084,8 @@ ruby_setenv(const char *name, const char *value)
     if (value) {
 	str = malloc(len += strlen(value) + 2);
 	snprintf(str, len, "%s=%s", name, value);
-	putenv(str);
+	if (putenv(str))
+	    rb_sys_fail("putenv");
     }
 #else  /* WIN32 */
     size_t len;