Author Helen Bailey joked with her alleged killer she would be scared if they argued because she did not "know what would happen", he told a court.
Her fiance Ian Stewart also claimed his bride-to-be took away his sedatives, which were eventually found in her system, saying: "This is something I might take."
The 56-year-old is on trial accused of slowly drugging his partner over the course of weeks and murdering her in a financially motivated plot.
Three months after she abruptly vanished on a spring afternoon last year, Ms Bailey, 51, was found dead, alongside her dachshund Boris, in a cesspit below the couple's garage.
Asked if the couple ever argued, Stewart told St Albans Crown Court: "No I can categorically say that because after we were in a relationship for six months Helen said to me: 'This isn't a proper relationship, we haven't had an argument'.
"She said 'I'm scared if we ever have an argument because I don't know what would happen, but I guarantee we will make up'."
Traces of the anti-insomnia medication Zopiclone were found in Ms Bailey during a post-mortem examination, which had been given to her partner by a doctor.
It is alleged Stewart had been doping her with the drugs and probably suffocated her when she was in a "stupefied" state.
"I just wasn't sleeping, as was Helen, I wasn't sleeping and we were getting more and more tired," he said.
"In this case, she heard what I was prescribed and she googled away as Helen always did with whatever came along and she almost instantly found that they should not be given to a patient with myasthenia gravis and took the tablets off me and said 'you're not taking those'.
On Tuesday, the court heard Stewart had been diagnosed with the muscular disorder in 1995 and had been out of work for decades.
He added: "Helen took them for herself and said 'this is something I might take' because she was having problems sleeping as well."
The defendant, of Baldock Road, Royston, Hertfordshire, denies murder, preventing a lawful burial, fraud and three counts of perverting the course of justice.
Stewart told the court that Ms Bailey had taken his sedatives on several occasions after confiscating them.
"Helen had a little bag which she put inside her handbag. In that bag there were things like cough tablets, but that is where she kept medicine - if we ever went anywhere she was ready for it," he said.
Asked if she had taken the Zopiclone, he replied: "I know definitely soon after she took them, if not the night I got them.
"I don't know whether I remember seeing her take them.
"She was when I was in hospital, she said she had taken them then."