Maybe you need to make the variable a number of some kind?
From the manual...
Modifier Operation
{I}{S}BIN{1..16} Send binary digits
{I}{S}DEC{1..5} Send decimal digits
{I}{S}HEX{1..4} Send hexadecimal digits
REP c\n Send character c repeated n times
STR ArrayVar{\n} Send string of n characters
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