diff --git a/common/iso_config.go b/common/iso_config.go
index ce7e7b223..655e92d98 100644
--- a/common/iso_config.go
+++ b/common/iso_config.go
@@ -23,18 +23,15 @@ type ISOConfig struct {
 	RawSingleISOUrl string   `mapstructure:"iso_url"`
 }
 
-func (c *ISOConfig) Prepare(ctx *interpolate.Context) ([]string, []error) {
-	// Validation
-	var errs []error
-	var err error
-	var warnings []string
-
+func (c *ISOConfig) Prepare(ctx *interpolate.Context) (warnings []string, errs []error) {
 	if c.RawSingleISOUrl == "" && len(c.ISOUrls) == 0 {
 		errs = append(
 			errs, errors.New("One of iso_url or iso_urls must be specified."))
+		return
 	} else if c.RawSingleISOUrl != "" && len(c.ISOUrls) > 0 {
 		errs = append(
 			errs, errors.New("Only one of iso_url or iso_urls may be specified."))
+		return
 	} else if c.RawSingleISOUrl != "" {
 		c.ISOUrls = []string{c.RawSingleISOUrl}
 	}
@@ -106,10 +103,12 @@ func (c *ISOConfig) Prepare(ctx *interpolate.Context) ([]string, []error) {
 	c.ISOChecksum = strings.ToLower(c.ISOChecksum)
 
 	for i, url := range c.ISOUrls {
-		c.ISOUrls[i], err = DownloadableURL(url)
+		url, err := DownloadableURL(url)
 		if err != nil {
 			errs = append(
 				errs, fmt.Errorf("Failed to parse iso_url %d: %s", i+1, err))
+		} else {
+			c.ISOUrls[i] = url
 		}
 	}
 
diff --git a/common/iso_config_test.go b/common/iso_config_test.go
index b7860d5f8..845e4e9dc 100644
--- a/common/iso_config_test.go
+++ b/common/iso_config_test.go
@@ -3,6 +3,8 @@ package common
 import (
 	"fmt"
 	"io/ioutil"
+	"net/http"
+	"net/http/httptest"
 	"reflect"
 	"testing"
 )
@@ -216,6 +218,15 @@ func TestISOConfigPrepare_ISOUrl(t *testing.T) {
 		t.Fatal("should have error")
 	}
 
+	// Test iso_url not set but checksum url is
+	ts := httptest.NewServer(http.FileServer(http.Dir("./test-fixtures/root")))
+	defer ts.Close()
+	i = testISOConfig()
+	i.RawSingleISOUrl = ""
+	i.ISOChecksum = ""
+	i.ISOChecksumURL = ts.URL + "/basic.txt"
+	warns, err = i.Prepare(nil)
+
 	// Test iso_url set
 	i = testISOConfig()
 	i.RawSingleISOUrl = "http://www.packer.io/the-OS.iso"