Problematic bc calculation in shell script

On Ubuntu 14.04.1 64-bit LTS I am writing a shell script and if I define the start of the sequence used in the for loop with a variable instead of a constant I get really weird behavior and there are lots of errors from the bc calculator. You can run the following code snippet to reproduce the errors:

#!/bin/bash
S=0.030
F=0.150
N=30
DIFF=`echo $F - $S | bc -l`
dw=`echo $DIFF / $N | bc -l`
is=`echo $S / $dw | bc -l`
if=`echo $F / $dw | bc -l`
for i in `seq $is $if` 
    do
        w=`echo "scale=3; $i * $dw" | bc -l`
done

If I change the start of the sequence to a constant it works perfectly, but when it is a variable there are problems the output is as follows:

(standard_in) 1: syntax error
...
...
(standard_in) 1: syntax error

What may be the reason behind this behaviour? I would like to receive your suggestions and comments.