diff --git a/api_test.go b/api_test.go
index cc27e5b09..cce4a251a 100644
--- a/api_test.go
+++ b/api_test.go
@@ -277,7 +277,7 @@ func TestManifestAPI(t *testing.T) {
 
 	resp = putManifest(t, "putting signed manifest", manifestURL, signedManifest)
 
-	checkResponse(t, "putting manifest", resp, http.StatusOK)
+	checkResponse(t, "putting signed manifest", resp, http.StatusOK)
 
 	resp, err = http.Get(manifestURL)
 	if err != nil {
@@ -299,9 +299,15 @@ func TestManifestAPI(t *testing.T) {
 }
 
 func putManifest(t *testing.T, msg, url string, v interface{}) *http.Response {
-	body, err := json.Marshal(v)
-	if err != nil {
-		t.Fatalf("unexpected error marshaling %v: %v", v, err)
+	var body []byte
+	if sm, ok := v.(*storage.SignedManifest); ok {
+		body = sm.Raw
+	} else {
+		var err error
+		body, err = json.MarshalIndent(v, "", "   ")
+		if err != nil {
+			t.Fatalf("unexpected error marshaling %v: %v", v, err)
+		}
 	}
 
 	req, err := http.NewRequest("PUT", url, bytes.NewReader(body))
diff --git a/storage/manifest.go b/storage/manifest.go
index 000bc617c..daeaa39b3 100644
--- a/storage/manifest.go
+++ b/storage/manifest.go
@@ -140,8 +140,9 @@ type SignedManifest struct {
 	Manifest
 
 	// Raw is the byte representation of the ImageManifest, used for signature
-	// verification. The manifest byte representation cannot change or it will
-	// have to be re-signed.
+	// verification. The value of Raw must be used directly during
+	// serialization, or the signature check will fail. The manifest byte
+	// representation cannot change or it will have to be re-signed.
 	Raw []byte `json:"-"`
 }
 
@@ -184,7 +185,9 @@ func (sm *SignedManifest) UnmarshalJSON(b []byte) error {
 }
 
 // MarshalJSON returns the contents of raw. If Raw is nil, marshals the inner
-// contents.
+// contents. Applications requiring a marshaled signed manifest should simply
+// use Raw directly, since the the content produced by json.Marshal will
+// compacted and will fail signature checks.
 func (sm *SignedManifest) MarshalJSON() ([]byte, error) {
 	if len(sm.Raw) > 0 {
 		return sm.Raw, nil