diff --git a/configure.ac b/configure.ac
index 2c4567c8..90495788 100644
--- a/configure.ac
+++ b/configure.ac
@@ -141,6 +141,7 @@ AC_PATH_PROG(w3m, w3m, false)
 AC_PATH_PROG(flex, flex, false)
 AC_PATH_PROG(bison, bison, false)
 NEED_PROG(perl, perl)
+NEED_PROG(sed, sed)
 NEED_PROG(tar, tar)
 AC_PATH_PROG(dot, dot)
 AC_PATH_PROG(dblatex, dblatex)
diff --git a/scripts/nix-prefetch-url.in b/scripts/nix-prefetch-url.in
index 7d8bd93a..31170fa9 100644
--- a/scripts/nix-prefetch-url.in
+++ b/scripts/nix-prefetch-url.in
@@ -24,7 +24,7 @@ fi
 # Handle escaped characters in the URI.  `+', `=' and `?' are the only
 # characters that are valid in Nix store path names but have a special
 # meaning in URIs.
-name=$(basename "$url" | sed -e 's/%2b/+/g' -e 's/%3d/=/g' -e 's/%3f/\?/g')
+name=$(basename "$url" | @sed@ -e 's/%2b/+/g' -e 's/%3d/=/g' -e 's/%3f/\?/g')
 if test -z "$name"; then echo "invalid url"; exit 1; fi
 
 
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index f4478a4d..d0d051f0 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1566,16 +1566,17 @@ void DerivationGoal::startBuilder()
         createDirs(chrootTmpDir);
         chmod(chrootTmpDir, 01777);
 
-        /* Create a /etc/passwd with entries for the build user and
-           the nobody account.  The latter is kind of a hack to
-           support Samba-in-QEMU. */
+        /* Create a /etc/passwd with entries for the build user and the
+           nobody account.  The latter is kind of a hack to support
+           Samba-in-QEMU.  */
         createDirs(chrootRootDir + "/etc");
 
         writeFile(chrootRootDir + "/etc/passwd",
             (format(
-                "nixbld:x:%1%:65534:Nix build user:/:/noshell\n"
+                "nixbld:x:%1%:%2%:Nix build user:/:/noshell\n"
                 "nobody:x:65534:65534:Nobody:/:/noshell\n")
-                % (buildUser.enabled() ? buildUser.getUID() : getuid())).str());
+                % (buildUser.enabled() ? buildUser.getUID() : getuid())
+                % (buildUser.enabled() ? buildUser.getGID() : getgid())).str());
 
         /* Bind-mount a user-configurable set of directories from the
            host file system.  The `/dev/pts' directory must be mounted
diff --git a/substitute.mk b/substitute.mk
index 430af16b..a6e00aab 100644
--- a/substitute.mk
+++ b/substitute.mk
@@ -16,6 +16,7 @@
 	 -e "s^@bzip2_bin_test\@^$(bzip2_bin_test)^g" \
 	 -e "s^@perl\@^$(perl)^g" \
 	 -e "s^@coreutils\@^$(coreutils)^g" \
+	 -e "s^@sed\@^$(sed)^g" \
 	 -e "s^@tar\@^$(tar)^g" \
 	 -e "s^@gzip\@^$(gzip)^g" \
 	 -e "s^@tr\@^$(tr)^g" \