This is involving an examination results slip issued in Joho's name, claiming he sat for his secondary school examinations at Serani High School in 1992.
Pius Macharia, Coast DCI boss, in a March 27 letter asked the governor to appear before a Samuel Nyambengi on Tuesday at 10am.
"I have reasons to believe Joho has information which may assist in completing investigations in a case of forgery," Macharia said.
"I do hereby require you for statement recording relating to the forgery of a 1992 result slip which is subject to this investigation."
In his response, Joho acknowledged receipt of the letter but said he had prior commitments and will not honor the summons.
He promised to appear for statement recording on Wednesday.
Joho has distanced himself from the allegations, saying he concluded his exams in 1993, and scored C+.
The social media has been awash with two different sets of result slips and certificates, with one indicating that he cleared in 1992 and got a D- (minus).
The other indicates that he cleared in 1993.
On Monday, two Mombasa residents interviewed at the school, claimed to have been Joho's classmates.
Joho's academic qualifications were challenged in court twice, but the court dismissed the cases against him.
-The Star