Oracle DB ORA-01722: invalid number error

I was getting a ORA-01722: invalid number error with the following code:

BigDecimal sequence = 1000595201
db.execute "alter sequence snp_calls_seq increment by ${sequence}"

Instead use :

BigDecimal sequence = 1000595201
String str = "alter sequence snp_calls_seq increment by ${sequence}"
db.execute str

Why ?