You don't have to go back to the Middle Ages to find people having difficulty with the concept of zero. A couple of more recent examples from a paper by Eric Hehner (http://www.cs.toronto.edu/%7Ehehner/RPUTP.pdf):
"In the 1991 Toronto phone book, there is a page that helpfully gives the time difference to various places in the world; to the U.K. it says "+5", and to Costa Rica it says "-1". But to Cuba it says "NA", and the legenda explains "time difference not applicable". By 1996 they tried to correct it; for Cuba it says "=", with the same explanation. In 1997 they discovered the number 0 , but they felt the need then, and still do today, to explain that 0 means "no time difference"
"The Fortran language of 1955 had a loop construct, but its body had to be executed at least once; I suppose it seemed senseless to have a loop whose body might be executed 0 times. The error was corrected in Algol in 1958, and in PL/I, and in Pascal, in part: iteration might be 0 times, but the data structure over which one is iterating, the array, had to have at least one element. In Pascal that meant there was no null string. And that put the algebra of data structures back where the algebra of natural numbers was prior to 1930."
"In the 1991 Toronto phone book, there is a page that helpfully gives the time difference to various places in the world; to the U.K. it says "+5", and to Costa Rica it says "-1". But to Cuba it says "NA", and the legenda explains "time difference not applicable". By 1996 they tried to correct it; for Cuba it says "=", with the same explanation. In 1997 they discovered the number 0 , but they felt the need then, and still do today, to explain that 0 means "no time difference"
"The Fortran language of 1955 had a loop construct, but its body had to be executed at least once; I suppose it seemed senseless to have a loop whose body might be executed 0 times. The error was corrected in Algol in 1958, and in PL/I, and in Pascal, in part: iteration might be 0 times, but the data structure over which one is iterating, the array, had to have at least one element. In Pascal that meant there was no null string. And that put the algebra of data structures back where the algebra of natural numbers was prior to 1930."