diff --git a/git-blame-someone-else b/git-blame-someone-else
index e34d7d0..11d9bb8 100644
--- a/git-blame-someone-else
+++ b/git-blame-someone-else
@@ -1,14 +1,15 @@
 #!/usr/bin/env bash
+set -e
 
-if [ $# -ne 2 ]; then
-  >&2 echo "Usage: $0 <author> <commit>"
+if [ $# -lt 1 ] || [ $# -gt 2 ]; then
+  >&2 echo "Usage: $0 <author> [commit]"
   exit 1
 fi
 
-AUTHOR=$1
+AUTHOR=${1:?}
 AUTHOR_NAME=$(echo $AUTHOR | perl -wlne '/^(.*?)\s*<.*>$/ and print $1')
 AUTHOR_EMAIL=$(echo $AUTHOR | perl -wlne '/^.*\s*<(.*)>$/ and print $1')
-COMMIT=$(git rev-parse --short $2)
+COMMIT=$(git rev-parse --short ${2:-HEAD})
 
 {
   GIT_SEQUENCE_EDITOR="sed -i -e 's/^pick $COMMIT/edit $COMMIT/'" git rebase -i $COMMIT~1^^