diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 3f595e3..37b839a 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2288,8 +2288,15 @@ void DerivationGoal::computeClosure()
                     % path % i->second.hashAlgo % printHash16or32(h) % printHash16or32(h2));
         }
 
-        /* Get rid of all weird permissions. */
-        canonicalisePathMetaData(path, buildUser.enabled() ? buildUser.getUID() : -1);
+	/* Get rid of all weird permissions.  If `path' already existed (as
+	   can happen with multiple-output derivations), it is already owned
+	   by `root' and not by the build user.  In that case, skip
+	   canonicalisation altogether.
+	   See <https://github.com/NixOS/nix/issues/122> for details.  */
+	canonicalisePathMetaData(path,
+				 (buildUser.enabled() && st.st_uid == buildUser.getUID())
+				 ? buildUser.getUID()
+				 : -1);
 
         /* For this output path, find the references to other paths
            contained in it.  Compute the SHA-256 NAR hash at the same