A question about DSA signing

[Michael Mi]

Hi, there,   I met some problem on OpenSSL library.   I used following statments to make signature.   ...  dsa=DSA_generate_parameters(512,seed,20,&counter,&h,dsa_cb,bio_err); ...  DSA_generate_key(dsa); ...  r = DSA_sign(0, str1, 20, sig, &siglen, dsa);   the result r equals to 1, that means good. But when I tried to sign some longer string, such as a string length of 21, the result is always bad. ( r equals to zero)   Can anyone tell me why?   Thanks.   Michael