diff --git a/entrypoint.postgres.sh.tmpl b/entrypoint.postgres.sh.tmpl
index 8174cda..cbd032b 100644
--- a/entrypoint.postgres.sh.tmpl
+++ b/entrypoint.postgres.sh.tmpl
@@ -30,16 +30,29 @@ if [ -f $PGDATA/PG_VERSION ]; then
     apt-get update && apt-get install -y --no-install-recommends \
       postgresql-$DATA_VERSION \
       && rm -rf /var/lib/apt/lists/*
+    # pg_upgrade must run as the old cluster's bootstrap superuser (the "install
+    # user", oid 10), and the new cluster must be initialised with that same
+    # user. It is not necessarily $POSTGRES_USER (e.g. clusters created with the
+    # default "postgres" superuser and a separate app role), so read it from the
+    # old cluster: briefly start it and ask, connecting as the app role we know.
+    PGBIN=/usr/lib/postgresql/$DATA_VERSION/bin
+    gosu postgres $PGBIN/pg_ctl -D $PGDATA -w \
+      -o "-c listen_addresses= -c unix_socket_directories=/tmp" start
+    INSTALL_USER=$(gosu postgres psql -h /tmp -U "$POSTGRES_USER" -d postgres -tAc \
+      "select rolname from pg_roles where oid = 10")
+    gosu postgres $PGBIN/pg_ctl -D $PGDATA -w stop
+    echo "old cluster install user: $INSTALL_USER"
     echo "shuffling around"
     gosu postgres mkdir $OLDDATA $NEWDATA
     chmod 700 $OLDDATA $NEWDATA
     mv $PGDATA/* $OLDDATA/ || true
     echo "running initdb"
-    # abuse entrypoint script for initdb by making server error out
-    gosu postgres bash -c "export PGDATA=$NEWDATA ; /usr/local/bin/docker-entrypoint.sh --invalid-arg || true"
+    # abuse entrypoint script for initdb by making server error out; initialise
+    # the new cluster with the same superuser as the old one so pg_upgrade matches
+    gosu postgres bash -c "export PGDATA=$NEWDATA POSTGRES_USER=$INSTALL_USER ; /usr/local/bin/docker-entrypoint.sh --invalid-arg || true"
     echo "running pg_upgrade"
     cd /tmp
-    gosu postgres pg_upgrade --link -b /usr/lib/postgresql/$DATA_VERSION/bin -d $OLDDATA -D $NEWDATA -U $POSTGRES_USER
+    gosu postgres pg_upgrade --link -b /usr/lib/postgresql/$DATA_VERSION/bin -d $OLDDATA -D $NEWDATA -U $INSTALL_USER
     cp $OLDDATA/pg_hba.conf $NEWDATA/
     mv $NEWDATA/* $PGDATA
     rm -rf $OLDDATA