fix a benchmark panic. see #1076
fix a benchmark panic. see #1076

raintank/metrictank,raintank/raintank-metric,raintank/raintank-metric,raintank/metrictank,replay/metrictank,replay/metrictank,raintank/metrictank,raintank/metrictank,raintank/raintank-metric,replay/metrictank,replay/metrictank