diff --git a/ChangeLog b/ChangeLog
index 23a93ed58e..080c2b3bc1 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+Tue Jan 26 21:36:22 2010  Tanaka Akira  <akr@fsij.org>
+
+	* configure.in: test unsetenv returns a value.
+	  unsetenv is void in older BSDs (FreeBSD 6 and OpenBSD 4.5 at least).
+
+	* hash.c (ruby_setenv): don't use the result of unsetenv if unsetenv
+	  doesn't return a value.
+
 Tue Jan 26 21:32:03 2010  Tanaka Akira  <akr@fsij.org>
 
 	* ext/socket/extconf.rb: suppress a warning.
diff --git a/configure.in b/configure.in
index 6709122613..791389f4b7 100644
--- a/configure.in
+++ b/configure.in
@@ -1121,6 +1121,16 @@ AC_CHECK_FUNCS(fmod killpg wait4 waitpid fork spawnv syscall chroot getcwd eacce
               mktime timegm gmtime_r clock_gettime gettimeofday\
               pread sendfile shutdown sigaltstack)
 
+AC_CACHE_CHECK(for unsetenv returns a value, rb_cv_unsetenv_return_value,
+  [AC_TRY_COMPILE([
+#include <stdlib.h>
+], [int v = unsetenv("foo");],
+	rb_cv_unsetenv_return_value=yes,
+	rb_cv_unsetenv_return_value=no)])
+if test "$rb_cv_unsetenv_return_value" = no; then
+  AC_DEFINE(VOID_UNSETENV)
+fi
+
 AC_CACHE_CHECK(for __builtin_setjmp, ac_cv_func___builtin_setjmp,
 [AC_TRY_LINK([@%:@include <setjmp.h>
     jmp_buf jb; void t(v) int v; {__builtin_longjmp(jb, v);}],
diff --git a/hash.c b/hash.c
index 124df01385..7a6bfe8134 100644
--- a/hash.c
+++ b/hash.c
@@ -2073,8 +2073,12 @@ ruby_setenv(const char *name, const char *value)
 	if (setenv(name, value, 1))
 	    rb_sys_fail("setenv");
     } else {
+#ifdef VOID_UNSETENV
+	unsetenv(name);
+#else
 	if (unsetenv(name))
 	    rb_sys_fail("unsetenv");
+#endif
     }
 #elif defined __sun__
     size_t len;