Mrs Jones

Mrs. Jones was reading a letter at breakfast. Suddenly she looked up suspiciously at her husband.
"Henry," she said, "I've just received a letter from mother saying she isn't accepting our invitation to come and stay, as we do not appear to want her. What does she mean by that? I told you to write and say that she was to come at her own convenience. You did write, didn`t you?"
"Er, yes, I did," said the husband. "But I couldn't spell `convenience, so I made it 'risk'."