diff --git a/src/Packet.cpp b/src/Packet.cpp
index 01b996e..86e64ff 100644
--- a/src/Packet.cpp
+++ b/src/Packet.cpp
@@ -26,10 +26,14 @@ namespace nbs {
 
         uint32_t subtype = 0;
         if (jsObject.Has("subtype")) {
-            subtype = jsObject.Get("subtype").As<Napi::Number>().Uint32Value();
+            if (jsObject.Get("subtype").IsNumber()) {
+                subtype = jsObject.Get("subtype").As<Napi::Number>().Uint32Value();
+            }
+            else if (!jsObject.Get("subtype").IsUndefined()) {
+                throw std::runtime_error("expected `subtype` to be a number");
+            }
         }
 
-        // Check types are valid
         uint64_t timestamp = 0;
         try {
             timestamp = timestamp::FromJsValue(jsObject.Get("timestamp"), env);
@@ -46,9 +50,6 @@ namespace nbs {
             throw std::runtime_error(std::string("error in `type`: ") + ex.what());
         }
 
-        if (!jsObject.Get("subtype").IsNumber()) {
-            throw std::runtime_error("expected `subtype` to be number");
-        }
         if (!jsObject.Get("payload").IsBuffer()) {
             throw std::runtime_error("expected `payload` to be buffer object");
         }
diff --git a/tests/test_encoder.js b/tests/test_encoder.js
index f91b18a..387de6b 100644
--- a/tests/test_encoder.js
+++ b/tests/test_encoder.js
@@ -118,6 +118,19 @@ test('NbsEncoder.write() throws for invalid arguments', () => {
       'NbsEncoder.write() throws with incorrect types for timestamp properties'
     );
 
+    assert.throws(
+      () => {
+        encoder.write({
+          timestamp: { seconds: 1897, nanos: 0 },
+          type: pingType,
+          subtype: 'string',
+          payload: Buffer.from('ping.699', 'utf8'),
+        });
+      },
+      /invalid type for argument `packet`: expected `subtype` to be a number/,
+      'NbsEncoder.write() throws with incorrect type for subtype'
+    );
+
     encoder.close();
   });
 });