diff --git a/SignalServiceKit/src/Messages/Attachments/TSAttachment.m b/SignalServiceKit/src/Messages/Attachments/TSAttachment.m
index 4bea23d36..c2d5c1565 100644
--- a/SignalServiceKit/src/Messages/Attachments/TSAttachment.m
+++ b/SignalServiceKit/src/Messages/Attachments/TSAttachment.m
@@ -31,6 +31,11 @@ NSUInteger const TSAttachmentSchemaVersion = 4;
         // This will fail with legacy iOS clients which don't upload attachment size.
         DDLogWarn(@"%@ Missing byteCount for attachment with serverId: %lld", self.tag, serverId);
     }
+    if (contentType.length < 1) {
+        DDLogWarn(@"%@ incoming attachment has invalid content type", self.tag);
+
+        contentType = OWSMimeTypeApplicationOctetStream;
+    }
     OWSAssert(contentType.length > 0);
 
     self = [super init];
@@ -55,6 +60,11 @@ NSUInteger const TSAttachmentSchemaVersion = 4;
                           byteCount:(UInt32)byteCount
                      sourceFilename:(nullable NSString *)sourceFilename
 {
+    if (contentType.length < 1) {
+        DDLogWarn(@"%@ outgoing attachment has invalid content type", self.tag);
+
+        contentType = OWSMimeTypeApplicationOctetStream;
+    }
     OWSAssert(contentType.length > 0);
     OWSAssert(byteCount > 0);
 
@@ -93,8 +103,14 @@ NSUInteger const TSAttachmentSchemaVersion = 4;
     _serverId = pointer.serverId;
     _encryptionKey = pointer.encryptionKey;
     _byteCount = pointer.byteCount;
-    _contentType = pointer.contentType;
     _sourceFilename = pointer.sourceFilename;
+    NSString *contentType = pointer.contentType;
+    if (contentType.length < 1) {
+        DDLogWarn(@"%@ incoming attachment has invalid content type", self.tag);
+
+        contentType = OWSMimeTypeApplicationOctetStream;
+    }
+    _contentType = contentType;
 
     _attachmentSchemaVersion = TSAttachmentSchemaVersion;
 
@@ -119,6 +135,12 @@ NSUInteger const TSAttachmentSchemaVersion = 4;
         OWSAssert(!_sourceFilename || [_sourceFilename isKindOfClass:[NSString class]]);
     }
 
+    if (_contentType.length < 1) {
+        DDLogWarn(@"%@ legacy attachment has invalid content type", self.tag);
+
+        _contentType = OWSMimeTypeApplicationOctetStream;
+    }
+
     return self;
 }
 
diff --git a/SignalServiceKit/src/Messages/Interactions/TSOutgoingMessage.m b/SignalServiceKit/src/Messages/Interactions/TSOutgoingMessage.m
index 0c783c7d2..2bc575a28 100644
--- a/SignalServiceKit/src/Messages/Interactions/TSOutgoingMessage.m
+++ b/SignalServiceKit/src/Messages/Interactions/TSOutgoingMessage.m
@@ -541,6 +541,7 @@ NSString *const kTSOutgoingMessageSentRecipientAll = @"kTSOutgoingMessageSentRec
 
     OWSSignalServiceProtosAttachmentPointerBuilder *builder = [OWSSignalServiceProtosAttachmentPointerBuilder new];
     [builder setId:attachmentStream.serverId];
+    OWSAssert(attachmentStream.contentType.length > 0);
     [builder setContentType:attachmentStream.contentType];
     [builder setFileName:filename];
     [builder setSize:attachmentStream.byteCount];