And each custom char take 8 bytes so you are consuming 152 * 8 = 1216 bytes.
A nano only has 2048 bytes of SRAM so you have used more than half already. When you compile your code, the compiler reports how large your sketch it as well as how much global variable space has been allocated.
Depending on what else you are doing, you may be pushing your memory limit... You might have to ditch the "easy-to-reference" alphabet which is a duplication of existing memory.