diff --git a/src/binding.cpp b/src/binding.cpp
index ff1bb59..60dffbc 100644
--- a/src/binding.cpp
+++ b/src/binding.cpp
@@ -237,6 +237,7 @@ struct MergeBaton : carmen::noncopyable {
     std::string pbf2;
     std::string pbf3;
     std::string method;
+    std::string error;
     Nan::Persistent<v8::Function> callback;
 };
 
@@ -251,156 +252,166 @@ void mergeQueue(uv_work_t* req) {
     std::map<uint64_t,bool> ids2;
 
     std::string merged;
-    protozero::pbf_writer writer(merged);
-
-    // Store ids from 1
-    protozero::pbf_reader pre1(pbf1);
-    while (pre1.next(CACHE_MESSAGE)) {
-        protozero::pbf_reader item = pre1.get_message();
-        while (item.next(CACHE_ITEM)) {
-            ids1.emplace(item.get_uint64(), true);
+    try {
+
+        protozero::pbf_writer writer(merged);
+
+        // Store ids from 1
+        protozero::pbf_reader pre1(pbf1);
+        while (pre1.next(CACHE_MESSAGE)) {
+            protozero::pbf_reader item = pre1.get_message();
+            while (item.next(CACHE_ITEM)) {
+                ids1.emplace(item.get_uint64(), true);
+            }
         }
-    }
 
-    // Store ids from 2
-    protozero::pbf_reader pre2(pbf2);
-    while (pre2.next(CACHE_MESSAGE)) {
-        protozero::pbf_reader item = pre2.get_message();
-        while (item.next(CACHE_ITEM)) {
-            ids2.emplace(item.get_uint64(), true);
+        // Store ids from 2
+        protozero::pbf_reader pre2(pbf2);
+        while (pre2.next(CACHE_MESSAGE)) {
+            protozero::pbf_reader item = pre2.get_message();
+            while (item.next(CACHE_ITEM)) {
+                ids2.emplace(item.get_uint64(), true);
+            }
         }
-    }
 
-    // No delta writes from message1
-    protozero::pbf_reader message1(pbf1);
-    while (message1.next(CACHE_MESSAGE)) {
-        protozero::pbf_writer item_writer(writer,1);
-        protozero::pbf_reader item = message1.get_message();
-        while (item.next(CACHE_ITEM)) {
-            uint64_t key_id = item.get_uint64();
+        // No delta writes from message1
+        protozero::pbf_reader message1(pbf1);
+        while (message1.next(CACHE_MESSAGE)) {
+            protozero::pbf_writer item_writer(writer,1);
+            protozero::pbf_reader item = message1.get_message();
+            while (item.next(CACHE_ITEM)) {
+                uint64_t key_id = item.get_uint64();
 
-            // Skip this id if also in message 2
-            if (ids2.find(key_id) != ids2.end()) break;
+                // Skip this id if also in message 2
+                if (ids2.find(key_id) != ids2.end()) break;
 
-            item_writer.add_uint64(1,key_id);
-            item.next();
-            protozero::packed_field_uint64 field{item_writer, 2};
-            auto vals = item.get_packed_uint64();
-            for (auto it = vals.first; it != vals.second; ++it) {
-                field.add_element(static_cast<uint64_t>(*it));
+                item_writer.add_uint64(1,key_id);
+                item.next();
+                protozero::packed_field_uint64 field{item_writer, 2};
+                auto vals = item.get_packed_uint64();
+                for (auto it = vals.first; it != vals.second; ++it) {
+                    field.add_element(static_cast<uint64_t>(*it));
+                }
             }
         }
-    }
 
-    // No delta writes from message2
-    protozero::pbf_reader message2(pbf2);
-    while (message2.next(CACHE_MESSAGE)) {
-        protozero::pbf_writer item_writer(writer,1);
-        protozero::pbf_reader item = message2.get_message();
-        while (item.next(CACHE_ITEM)) {
-            uint64_t key_id = item.get_uint64();
+        // No delta writes from message2
+        protozero::pbf_reader message2(pbf2);
+        while (message2.next(CACHE_MESSAGE)) {
+            protozero::pbf_writer item_writer(writer,1);
+            protozero::pbf_reader item = message2.get_message();
+            while (item.next(CACHE_ITEM)) {
+                uint64_t key_id = item.get_uint64();
 
-            // Skip this id if also in message 2
-            if (ids1.find(key_id) != ids1.end()) break;
+                // Skip this id if also in message 2
+                if (ids1.find(key_id) != ids1.end()) break;
 
-            item_writer.add_uint64(1,key_id);
-            item.next();
-            protozero::packed_field_uint64 field{item_writer, 2};
-            auto vals = item.get_packed_uint64();
-            for (auto it = vals.first; it != vals.second; ++it) {
-                field.add_element(static_cast<uint64_t>(*it));
+                item_writer.add_uint64(1,key_id);
+                item.next();
+                protozero::packed_field_uint64 field{item_writer, 2};
+                auto vals = item.get_packed_uint64();
+                for (auto it = vals.first; it != vals.second; ++it) {
+                    field.add_element(static_cast<uint64_t>(*it));
+                }
             }
         }
-    }
 
-    // Delta writes for ids in both message1 and message2
-    protozero::pbf_reader overlap1(pbf1);
-    while (overlap1.next(CACHE_MESSAGE)) {
-        protozero::pbf_writer item_writer(writer,1);
-        protozero::pbf_reader item = overlap1.get_message();
-        while (item.next(CACHE_ITEM)) {
-            uint64_t key_id = item.get_uint64();
-
-            // Skip ids that are only in one or the other lists
-            if (ids1.find(key_id) == ids1.end() || ids2.find(key_id) == ids2.end()) break;
-
-            item_writer.add_uint64(1,key_id);
-
-            item.next();
-            uint64_t lastval = 0;
-            Cache::intarray varr;
-
-            // Add values from pbf1
-            auto vals = item.get_packed_uint64();
-            for (auto it = vals.first; it != vals.second; ++it) {
-                if (method == "freq") {
-                    varr.emplace_back(*it);
-                    break;
-                } else if (lastval == 0) {
-                    lastval = *it;
-                    varr.emplace_back(lastval);
-                } else {
-                    lastval = lastval - *it;
-                    varr.emplace_back(lastval);
+        // Delta writes for ids in both message1 and message2
+        protozero::pbf_reader overlap1(pbf1);
+        while (overlap1.next(CACHE_MESSAGE)) {
+            protozero::pbf_writer item_writer(writer,1);
+            protozero::pbf_reader item = overlap1.get_message();
+            while (item.next(CACHE_ITEM)) {
+                uint64_t key_id = item.get_uint64();
+
+                // Skip ids that are only in one or the other lists
+                if (ids1.find(key_id) == ids1.end() || ids2.find(key_id) == ids2.end()) break;
+
+                item_writer.add_uint64(1,key_id);
+
+                item.next();
+                uint64_t lastval = 0;
+                Cache::intarray varr;
+
+                // Add values from pbf1
+                auto vals = item.get_packed_uint64();
+                for (auto it = vals.first; it != vals.second; ++it) {
+                    if (method == "freq") {
+                        varr.emplace_back(*it);
+                        break;
+                    } else if (lastval == 0) {
+                        lastval = *it;
+                        varr.emplace_back(lastval);
+                    } else {
+                        lastval = lastval - *it;
+                        varr.emplace_back(lastval);
+                    }
                 }
-            }
 
-            // Check pbf2 for this id and merge its items if found
-            protozero::pbf_reader overlap2(pbf2);
-            while (overlap2.next(CACHE_MESSAGE)) {
-                protozero::pbf_reader item2 = overlap2.get_message();
-                while (item2.next(CACHE_ITEM)) {
-                    uint64_t key_id2 = item2.get_uint64();
-                    if (key_id2 != key_id) break;
-                    item2.next();
-                    lastval = 0;
-                    auto vals2 = item2.get_packed_uint64();
-                    for (auto it = vals2.first; it != vals2.second; ++it) {
-                        if (method == "freq") {
-                            if (key_id2 == 1) {
-                                varr[0] = varr[0] > *it ? varr[0] : *it;
+                // Check pbf2 for this id and merge its items if found
+                protozero::pbf_reader overlap2(pbf2);
+                while (overlap2.next(CACHE_MESSAGE)) {
+                    protozero::pbf_reader item2 = overlap2.get_message();
+                    while (item2.next(CACHE_ITEM)) {
+                        uint64_t key_id2 = item2.get_uint64();
+                        if (key_id2 != key_id) break;
+                        item2.next();
+                        lastval = 0;
+                        auto vals2 = item2.get_packed_uint64();
+                        for (auto it = vals2.first; it != vals2.second; ++it) {
+                            if (method == "freq") {
+                                if (key_id2 == 1) {
+                                    varr[0] = varr[0] > *it ? varr[0] : *it;
+                                } else {
+                                    varr[0] = varr[0] + *it;
+                                }
+                                break;
+                            } else if (lastval == 0) {
+                                lastval = *it;
+                                varr.emplace_back(lastval);
                             } else {
-                                varr[0] = varr[0] + *it;
+                                lastval = lastval - *it;
+                                varr.emplace_back(lastval);
                             }
-                            break;
-                        } else if (lastval == 0) {
-                            lastval = *it;
-                            varr.emplace_back(lastval);
-                        } else {
-                            lastval = lastval - *it;
-                            varr.emplace_back(lastval);
                         }
                     }
                 }
-            }
 
-            // Sort for proper delta encoding
-            std::sort(varr.begin(), varr.end(), std::greater<uint64_t>());
-
-            // Write varr to merged protobuf
-            protozero::packed_field_uint64 field{item_writer, 2};
-            lastval = 0;
-            for (auto const& vitem : varr) {
-                if (lastval == 0) {
-                    field.add_element(static_cast<uint64_t>(vitem));
-                } else {
-                    field.add_element(static_cast<uint64_t>(lastval - vitem));
+                // Sort for proper delta encoding
+                std::sort(varr.begin(), varr.end(), std::greater<uint64_t>());
+
+                // Write varr to merged protobuf
+                protozero::packed_field_uint64 field{item_writer, 2};
+                lastval = 0;
+                for (auto const& vitem : varr) {
+                    if (lastval == 0) {
+                        field.add_element(static_cast<uint64_t>(vitem));
+                    } else {
+                        field.add_element(static_cast<uint64_t>(lastval - vitem));
+                    }
+                    lastval = vitem;
                 }
-                lastval = vitem;
             }
         }
-    }
 
-    baton->pbf3 = merged;
+        baton->pbf3 = merged;
+    } catch (std::exception const& ex) {
+        baton->error = ex.what();
+    }
 }
 
 void mergeAfter(uv_work_t* req) {
     Nan::HandleScope scope;
     MergeBaton *baton = static_cast<MergeBaton *>(req->data);
-    std::string const& merged = baton->pbf3;
-    Local<Object> buf = Nan::CopyBuffer((char*)merged.data(), merged.size()).ToLocalChecked();
-    Local<Value> argv[2] = { Nan::Null(), buf };
-    Nan::MakeCallback(Nan::GetCurrentContext()->Global(), Nan::New(baton->callback), 2, argv);
+    if (!baton->error.empty()) {
+        v8::Local<v8::Value> argv[1] = { Nan::Error(baton->error.c_str()) };
+        Nan::MakeCallback(Nan::GetCurrentContext()->Global(), Nan::New(baton->callback), 1, argv);
+    } else {
+        std::string const& merged = baton->pbf3;
+        Local<Object> buf = Nan::CopyBuffer((char*)merged.data(), merged.size()).ToLocalChecked();
+        Local<Value> argv[2] = { Nan::Null(), buf };
+        Nan::MakeCallback(Nan::GetCurrentContext()->Global(), Nan::New(baton->callback), 2, argv);
+    }
     baton->callback.Reset();
     delete baton;
 }
diff --git a/test/merge.test.js b/test/merge.test.js
index 82cc098..d13aae3 100644
--- a/test/merge.test.js
+++ b/test/merge.test.js
@@ -56,6 +56,9 @@ tape('#merge invalid pbf throws JS error', function(assert) {
     var cacheA = new Cache('a');
     cacheA.merge(new Buffer("phony protobuf"), new Buffer("phony protobuf"), 'freq', function(err, merged) {
         assert.ok(err);
+        if (err) {
+            assert.ok(err.message.indexOf("unknown pbf field type exception") > -1);            
+        }
         assert.end();
     })
 });