What value can cause this error?

I got this error message from a customer and no further information is
available.
What could have caused this error?
integer value too large to represent
    while executing
"expr {round($val*(10.0**($decimals+3)))/(10.0**$decimals)}"
I know that "val" is a proper number and that "decimals" is 6.
I tried to run this expression with very high "val" values and could not
reproduce the error.
Many thanks
Alexandru