diff --git a/cabal-graphdeps.cabal b/cabal-graphdeps.cabal
--- a/cabal-graphdeps.cabal
+++ b/cabal-graphdeps.cabal
@@ -1,5 +1,5 @@
 name: cabal-graphdeps
-version: 0.1.1
+version: 0.1.2
 license: MIT
 license-file: license.txt
 author: John Millikin <john@john-millikin.com>
@@ -21,7 +21,7 @@
 source-repository this
   type: git
   location: https://john-millikin.com/code/cabal-graphdeps/
-  tag: cabal-graphdeps_0.1.1
+  tag: cabal-graphdeps_0.1.2
 
 executable cabal-graphdeps
   main-is: Main.hs
diff --git a/src/Main.hs b/src/Main.hs
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -16,6 +16,7 @@
 import qualified Data.Set as Set
 import           Options
 import qualified System.Directory as Filesystem
+import           System.Exit (exitFailure)
 import qualified System.Process as Process
 import qualified System.IO as IO
 import           System.IO.Temp (withSystemTempDirectory)
@@ -170,9 +171,14 @@
 
 main :: IO ()
 main = runCommand $ \opts args -> do
+	rootPackageName <- case args of
+		[x] -> return x
+		_ -> do
+			IO.hPutStrLn IO.stderr "Usage: cabal-graphdeps <package-name>"
+			exitFailure
 	deps <- withSystemTempDirectory "cabal-graphdeps.d-" $ \dir -> withCurrentDirectory dir $ do
 		initSandbox opts
-		resolveDeps opts Map.empty (args !! 0)
+		resolveDeps opts Map.empty rootPackageName
 	putStrLn "digraph {"
-	printDeps opts deps (args !! 0)
+	printDeps opts deps rootPackageName
 	putStrLn "}"