diff --git a/k9mail/src/main/java/com/fsck/k9/crypto/MessageCryptoStructureDetector.java b/k9mail/src/main/java/com/fsck/k9/crypto/MessageCryptoStructureDetector.java
index 219778f27..974811957 100644
--- a/k9mail/src/main/java/com/fsck/k9/crypto/MessageCryptoStructureDetector.java
+++ b/k9mail/src/main/java/com/fsck/k9/crypto/MessageCryptoStructureDetector.java
@@ -227,8 +227,10 @@ public class MessageCryptoStructureDetector {
         }
 
         String protocolParameter = MimeUtility.getHeaderParameter(part.getContentType(), PROTOCOL_PARAMETER);
-        BodyPart signatureBodyPart = mimeMultipart.getBodyPart(1);
-        return isSameMimeType(protocolParameter, signatureBodyPart.getMimeType());
+
+        boolean dataUnavailable = protocolParameter == null && mimeMultipart.getBodyPart(0).getBody() == null;
+        boolean protocolMatches = isSameMimeType(protocolParameter, mimeMultipart.getBodyPart(1).getMimeType());
+        return dataUnavailable || protocolMatches;
     }
 
     private static boolean isPartMultipartEncrypted(Part part) {
@@ -244,8 +246,10 @@ public class MessageCryptoStructureDetector {
         }
 
         String protocolParameter = MimeUtility.getHeaderParameter(part.getContentType(), PROTOCOL_PARAMETER);
-        BodyPart signatureBodyPart = mimeMultipart.getBodyPart(0);
-        return isSameMimeType(protocolParameter, signatureBodyPart.getMimeType());
+
+        boolean dataUnavailable = protocolParameter == null && mimeMultipart.getBodyPart(1).getBody() == null;
+        boolean protocolMatches = isSameMimeType(protocolParameter, mimeMultipart.getBodyPart(0).getMimeType());
+        return dataUnavailable || protocolMatches;
     }
 
     public static boolean isMultipartEncryptedOpenPgpProtocol(Part part) {
diff --git a/k9mail/src/test/java/com/fsck/k9/crypto/MessageCryptoStructureDetectorTest.java b/k9mail/src/test/java/com/fsck/k9/crypto/MessageCryptoStructureDetectorTest.java
index 352d7bed3..077226597 100644
--- a/k9mail/src/test/java/com/fsck/k9/crypto/MessageCryptoStructureDetectorTest.java
+++ b/k9mail/src/test/java/com/fsck/k9/crypto/MessageCryptoStructureDetectorTest.java
@@ -182,7 +182,7 @@ public class MessageCryptoStructureDetectorTest {
         Message message = messageFromBody(
                 multipart("encrypted", "application/not-pgp-encrypted",
                         bodypart("application/pgp-encrypted"),
-                        bodypart("application/octet-stream")
+                        bodypart("application/octet-stream", "content")
                 )
         );
 
@@ -192,7 +192,7 @@ public class MessageCryptoStructureDetectorTest {
     }
 
     @Test
-    public void findEncrypted__withEmptyProtocol__shouldReturnEmpty() throws Exception {
+    public void findEncrypted__withBadProtocolAndNoBody__shouldReturnRoot() throws Exception {
         Message message = messageFromBody(
                 multipart("encrypted", null,
                         bodypart("application/pgp-encrypted"),
@@ -202,6 +202,21 @@ public class MessageCryptoStructureDetectorTest {
 
         List<Part> encryptedParts = MessageCryptoStructureDetector.findMultipartEncryptedParts(message);
 
+        assertEquals(1, encryptedParts.size());
+        assertSame(message, encryptedParts.get(0));
+    }
+
+    @Test
+    public void findEncrypted__withEmptyProtocol__shouldReturnEmpty() throws Exception {
+        Message message = messageFromBody(
+                multipart("encrypted", null,
+                        bodypart("application/pgp-encrypted"),
+                        bodypart("application/octet-stream", "content")
+                )
+        );
+
+        List<Part> encryptedParts = MessageCryptoStructureDetector.findMultipartEncryptedParts(message);
+
         assertTrue(encryptedParts.isEmpty());
     }
 
@@ -324,10 +339,26 @@ public class MessageCryptoStructureDetectorTest {
     }
 
     @Test
-    public void findSigned__withBadProtocol__shouldReturnRoot() throws Exception {
+    public void findSigned__withNoProtocolAndNoBody__shouldReturnRoot() throws Exception {
+        Message message = messageFromBody(
+                multipart("signed", null,
+                        bodypart("text/plain"),
+                        bodypart("application/pgp-signature")
+                )
+        );
+
+        List<Part> signedParts = MessageCryptoStructureDetector
+                .findMultipartSignedParts(message, messageCryptoAnnotations);
+
+        assertEquals(1, signedParts.size());
+        assertSame(message, signedParts.get(0));
+    }
+
+    @Test
+    public void findSigned__withBadProtocol__shouldReturnNothing() throws Exception {
         Message message = messageFromBody(
                 multipart("signed", "application/not-pgp-signature",
-                        bodypart("text/plain"),
+                        bodypart("text/plain", "content"),
                         bodypart("application/pgp-signature")
                 )
         );
@@ -342,7 +373,7 @@ public class MessageCryptoStructureDetectorTest {
     public void findSigned__withEmptyProtocol__shouldReturnRoot() throws Exception {
         Message message = messageFromBody(
                 multipart("signed", null,
-                        bodypart("text/plain"),
+                        bodypart("text/plain", "content"),
                         bodypart("application/pgp-signature")
                 )
         );