Kill: 5656: Arguments Must Be Process or Job Ids

Having redis-server ON (launched with redis-server &).

Running this command in a terminal works perfectly:

kill -s SIGTERM "`pgrep redis-server`"

But in a script it outputs the following message and does not kill the process:

myscript.sh: line 17: kill: 1448
1452: arguments must be process or job IDs

(if I do: pgrep redis-server in this example it will outputs me 1448)

My complete source script:

#!/bin/bash

if [ -a "redis-server_must_be_ON" ]
then
  if [ "`redis-cli PING`" != "PONG" ]
  then
    redis-server &
    if [ "`redis-cli PING`" != "PONG" ]
    then
      echo "redis-server still not running while it should have been set on." >> /dev/stderr
      exit 1
    fi
  fi
else
  if [ "`redis-cli PING`" == "PONG" ]
  then
    kill -s SIGTERM "`pgrep redis-server`"
    if [ "`redis-cli PING`" == "PONG" ]
    then
      echo "redis-server still running while it should have been set off." >> /dev/stderr
      exit 1
    fi
  fi
fi

(Here if I replace the pgrep redis-server with some sort of mascarade like pgrep bash1.sh it also works fine).

Is my script correct, what do I miss ?