diff --git a/lib/specs/spec_1.js b/lib/specs/spec_1.js
index 6944fea..3b4207e 100644
--- a/lib/specs/spec_1.js
+++ b/lib/specs/spec_1.js
@@ -1,11 +1,20 @@
 const uuid  = require("uuid/v4");
 const empty = require("is-empty");
 const Ajv   = require("ajv");
+const URI   = require("uri-js");
 
 const {
-  asData
+  asData,
+  isBoolean,
+  isInteger,
+  isString,
+  isDate,
+  isBinary
 } = require("../utils/fun.js");
 
+const isValidType = (v) =>
+  (isBoolean(v) || isInteger(v) || isString(v) || isDate(v) || isBinary(v));
+
 const RESERVED_ATTRIBUTES = {
   type: "type",
   specversion: "specversion",
@@ -165,7 +174,11 @@ Spec1.prototype.getData = function() {
 
 Spec1.prototype.addExtension = function(key, value){
   if(!RESERVED_ATTRIBUTES.hasOwnProperty(key)){
-    this.payload[key] = value;
+    if(isValidType(value)){
+      this.payload[key] = value;
+    } else {
+      throw {message: "Invalid type of extension value"};
+    }
   } else {
     throw {message: "Reserved attribute name: '" + key + "'"};
   }