diff --git a/ChangeLog b/ChangeLog
index 55d08c087a..dd962f8988 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Jul 30 08:51:51 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* file.c (file_expand_path): home directory must be absolute.
+	  [ruby-core:31537]
+
 Fri Jul 30 08:33:20 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* file.c (file_expand_path): should check if could find user.
diff --git a/file.c b/file.c
index 355f8e8a8a..46d75498dc 100644
--- a/file.c
+++ b/file.c
@@ -2851,6 +2851,7 @@ file_expand_path(VALUE fname, VALUE dname, int abs_mode, VALUE result)
     tainted = OBJ_TAINTED(fname);
 
     if (s[0] == '~' && abs_mode == 0) {      /* execute only if NOT absolute_path() */
+	long userlen = 0;
 	tainted = 1;
 	if (isdirsep(s[1]) || s[1] == '\0') {
 	    buf = 0;
@@ -2859,15 +2860,24 @@ file_expand_path(VALUE fname, VALUE dname, int abs_mode, VALUE result)
 	}
 	else {
 	    s = nextdirsep(b = s);
-	    BUFCHECK(bdiff + (s-b) >= buflen);
-	    memcpy(p, b, s-b);
-	    rb_str_set_len(result, s-b);
+	    userlen = s - b;
+	    BUFCHECK(bdiff + userlen >= buflen);
+	    memcpy(p, b, userlen);
+	    rb_str_set_len(result, userlen);
 	    buf = p + 1;
-	    p += s-b;
+	    p += userlen;
 	}
 	if (NIL_P(rb_home_dir(buf, result))) {
 	    rb_raise(rb_eArgError, "can't find user %s", buf);
 	}
+	if (!rb_is_absolute_path(RSTRING_PTR(result))) {
+	    if (userlen) {
+		rb_raise(rb_eArgError, "non-absolute home of %.*s", (int)userlen, b);
+	    }
+	    else {
+		rb_raise(rb_eArgError, "non-absolute home");
+	    }
+	}
 	BUFINIT();
 	p = pend;
     }
diff --git a/test/ruby/test_file_exhaustive.rb b/test/ruby/test_file_exhaustive.rb
index ee4df6b154..5344494679 100644
--- a/test/ruby/test_file_exhaustive.rb
+++ b/test/ruby/test_file_exhaustive.rb
@@ -397,6 +397,18 @@ class TestFileExhaustive < Test::Unit::TestCase
     assert_kind_of(String, File.expand_path("~"))
     assert_raise(ArgumentError) { File.expand_path("~foo_bar_baz_unknown_user_wahaha") }
     assert_raise(ArgumentError) { File.expand_path("~foo_bar_baz_unknown_user_wahaha", "/") }
+    begin
+      bug3630 = '[ruby-core:31537]'
+      home = ENV["HOME"]
+      ENV["HOME"] = nil
+      assert_raise(ArgumentError) { File.expand_path("~") }
+      ENV["HOME"] = "~"
+      assert_raise(ArgumentError, bug3630) { File.expand_path("~") }
+      ENV["HOME"] = "."
+      assert_raise(ArgumentError, bug3630) { File.expand_path("~") }
+    ensure
+      ENV["HOME"] = home
+    end
     assert_incompatible_encoding {|d| File.expand_path(d)}
   end