diff --git a/src/gmail.js b/src/gmail.js
index 284b7593..366b8414 100755
--- a/src/gmail.js
+++ b/src/gmail.js
@@ -3587,6 +3587,9 @@ var Gmail = function(localJQuery) {
      * @param email_id: new style email id. Legacy IDs not supported. If empty, default to latest in view.
      */
     api.new.get.email_data = function(email_id) {
+        if (!email_id.statsWith("msg")) {
+            throw new Error("Legacy email-ID used in new API!");
+        }
         email_id = email_id || api.new.get.email_id();
         return api.cache.emailIdCache[email_id];
     };
@@ -3597,6 +3600,10 @@ var Gmail = function(localJQuery) {
      * @param thread_id: new style thread id. Legacy IDs not supported. If empty, default to current.
      */
     api.new.get.thread_data = function(thread_id) {
+        if (!thread_id.statsWith("thread")) {
+            throw new Error("Legacy email-ID used where new-type thread-id expected!");
+        }
+
         thread_id = thread_id || api.new.get.thread_id();
         return api.cache.threadCache[thread_id];
     };
diff --git a/test/test.new.js b/test/test.new.js
new file mode 100644
index 00000000..655295cd
--- /dev/null
+++ b/test/test.new.js
@@ -0,0 +1,28 @@
+"use strict";
+let assert = require('assert');
+let Gmail = require('../src/gmail').Gmail;
+let gmail = new Gmail();
+
+describe("gmail.new.get", () => {
+    it("email_data() rejects legacy-style IDs", () => {
+        let failed = false;
+        try {
+            let res = gmail.new.get.email_data("16a0d1f820d515e2");
+        } catch (err) {
+            failed = true;
+        }
+
+        assert.equal(true, failed);
+    });
+
+    it("thread_data() rejects legacy-style IDs", () => {
+        let failed = false;
+        try {
+            let res = gmail.new.get.thread_data("16a0d1f820d515e2");
+        } catch (err) {
+            failed = true;
+        }
+
+        assert.equal(true, failed);
+    });
+});