diff --git a/dockertar-sha256-helper.go b/dockertar-sha256-helper.go
index 4a4498ff..ac020af1 100644
--- a/dockertar-sha256-helper.go
+++ b/dockertar-sha256-helper.go
@@ -1,15 +1,21 @@
 package main
 
 import (
-	"fmt"
 	"compress/gzip"
-	"os"
-	"io"
 	"crypto/sha256"
+	"fmt"
+	"io"
+	"os"
 )
 
 func main() {
+	// Require one positional argument
+	if len(os.Args) != 2 {
+		panic("One positional argument is required")
+	}
 	reader, err := os.Open(os.Args[1])
+	// Close the input file at the end of the function
+	defer reader.Close()
 	if err != nil {
 		os.Exit(1)
 	}
@@ -29,7 +35,8 @@ func main() {
 			panic(err)
 		}
 	}
+	// We must explicitly close before executing the sum
+	// as defer will cause an incorrect result
 	w.Close()
-
 	fmt.Printf("%x\n", sha_256.Sum(nil))
 }