error in conv function

in matlab ,i wrote a code like this

v=-20:100;
im=(v==0);
b=[2 0 0];
g=conv(im,b)

i get the following error,

??? Error using ==> conv2
First and second arguments must be single or double.

Error in ==> conv at 40
c = conv2(a),b),shape);

Error in ==> del at 4
g=conv(im,b)


please tell me why...? am i wrong??

thanks in advance... please tell me as early as possible...

the question may be very silly... but i cant understand it anyway.. so please help..

I've tried your program with Matlab R2008a.

For
g=conv(im,b)
it works fine.

For
g=conv2(im,b)
I get the same error.

But if I do this
g=conv2(double(im),b)
then it gives me no error, and the result is
g =
Columns 1 through 18
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Columns 19 through 36
0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Columns 37 through 54
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Columns 55 through 72
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Columns 73 through 90
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Columns 91 through 108
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Columns 109 through 123
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0