diff --git a/src/serialization/sb2.js b/src/serialization/sb2.js
index 5ce617c2e..d9dced0f7 100644
--- a/src/serialization/sb2.js
+++ b/src/serialization/sb2.js
@@ -134,19 +134,17 @@ const parseBlockList = function (blockList, addBroadcastMsg, getVariableId, exte
         const parsedBlockAndComments = parseBlock(block, addBroadcastMsg, getVariableId,
             extensions, comments, commentIndex);
 
-        if (parsedBlockAndComments) { // Could fail due to unrecognized op-codes
-            const parsedBlock = parsedBlockAndComments[0];
-            // Update commentIndex
-            commentIndex = parsedBlockAndComments[1];
+        const parsedBlock = parsedBlockAndComments[0];
+        // Update commentIndex
+        commentIndex = parsedBlockAndComments[1];
 
-            if (typeof parsedBlock === 'undefined') continue;
-            if (previousBlock) {
-                parsedBlock.parent = previousBlock.id;
-                previousBlock.next = parsedBlock.id;
-            }
-            previousBlock = parsedBlock;
-            resultingList.push(parsedBlock);
+        if (!parsedBlock) continue;
+        if (previousBlock) {
+            parsedBlock.parent = previousBlock.id;
+            previousBlock.next = parsedBlock.id;
         }
+        previousBlock = parsedBlock;
+        resultingList.push(parsedBlock);
     }
     return [resultingList, commentIndex];
 };
@@ -709,9 +707,19 @@ const specMapBlock = function (block) {
  * and second item is the updated comment index (after this block and its children are parsed)
  */
 const parseBlock = function (sb2block, addBroadcastMsg, getVariableId, extensions, comments, commentIndex) {
+    const commentsForParsedBlock = (comments && typeof commentIndex === 'number' && !isNaN(commentIndex)) ?
+        comments[commentIndex] : null;
     const blockMetadata = specMapBlock(sb2block);
     if (!blockMetadata) {
-        return;
+        // No block opcode found, exclude this block, increment the comment id and
+        // send all linked comments back to zero/zero to prevent serialization issues.
+        if (commentsForParsedBlock) {
+            commentsForParsedBlock.forEach(comment => {
+                comment.blockId = null;
+                comment.x = comment.y = 0;
+            });
+        }
+        return [null, commentIndex + 1];
     }
     const oldOpcode = sb2block[0];
 
@@ -734,15 +742,18 @@ const parseBlock = function (sb2block, addBroadcastMsg, getVariableId, extension
     };
 
     // Attach any comments to this block..
-    const commentsForParsedBlock = (comments && typeof commentIndex === 'number' && !isNaN(commentIndex)) ?
-        comments[commentIndex] : null;
     if (commentsForParsedBlock) {
-        // TODO currently only attaching the last comment to the block if there are multiple...
-        // not sure what to do here.. concatenate all the messages in all the comments and only
-        // keep one around?
+        // Attach only the last comment to the block, make all others workspace comments
         activeBlock.comment = commentsForParsedBlock[commentsForParsedBlock.length - 1].id;
         commentsForParsedBlock.forEach(comment => {
-            comment.blockId = activeBlock.id;
+            if (comment.id === activeBlock.comment) {
+                comment.blockId = activeBlock.id;
+            } else {
+                // All other comments don't get a block ID and are sent back to zero.
+                // This is important, because if they have `null` x/y, serialization breaks.
+                comment.blockId = null;
+                comment.x = comment.y = 0;
+            }
         });
     }
     commentIndex++;
diff --git a/test/fixtures/unknown-opcode.sb2 b/test/fixtures/unknown-opcode.sb2
index 81e191b46..cc3341aa5 100644
Binary files a/test/fixtures/unknown-opcode.sb2 and b/test/fixtures/unknown-opcode.sb2 differ
diff --git a/test/integration/unknown-opcode.js b/test/integration/unknown-opcode.js
index d7a5519f6..53b9b64f9 100644
--- a/test/integration/unknown-opcode.js
+++ b/test/integration/unknown-opcode.js
@@ -31,6 +31,24 @@ test('unknown opcode', t => {
         t.equal(blocks.getBlock(topBlockId).opcode, 'sound_play');
         t.equal(blocks.getBlock(secondBlockId).opcode, 'sound_play');
         t.equal(thirdBlockId, null);
+
+        const target = vm.runtime.targets[0];
+        const topCommentId = blocks.getBlock(topBlockId).comment;
+        const secondCommentId = blocks.getBlock(secondBlockId).comment;
+
+        t.equal(target.comments[topCommentId].text, 'pop1 comment');
+        t.equal(target.comments[secondCommentId].text, 'pop2 comment');
+
+        // The comment previously attached to the undefined block should become
+        // a workspace comment, at 0/0, with the same text as it had.
+        const undefinedCommentId = Object.keys(target.comments).filter(id =>
+            id !== topCommentId && id !== secondCommentId)[0];
+        const undefinedComment = target.comments[undefinedCommentId];
+        t.equal(undefinedComment.blockId, null);
+        t.equal(undefinedComment.text, 'undefined comment');
+        t.equal(undefinedComment.x, 0);
+        t.equal(undefinedComment.y, 0);
+
         t.end();
         process.nextTick(process.exit);
     });