diff --git a/google/protobuf/LICENSE b/google/LICENSE
similarity index 100%
rename from google/protobuf/LICENSE
rename to google/LICENSE
diff --git a/google/protobuf/README.md b/google/README.md
similarity index 100%
rename from google/protobuf/README.md
rename to google/README.md
diff --git a/src/decoder.js b/src/decoder.js
index 9dcb10bff..25202c8ed 100644
--- a/src/decoder.js
+++ b/src/decoder.js
@@ -95,7 +95,7 @@ function decoder(mtype) {
         var rfield = mtype._fieldsArray[i];
         if (rfield.required) gen
     ("if(!m.hasOwnProperty(%j))", rfield.name)
-        ("throw util.ProtocolError(%j,m)", missing(rfield));
+        ("throw util.ProtocolError(%j,{instance:m})", missing(rfield));
     }
 
     return gen
diff --git a/src/util/minimal.js b/src/util/minimal.js
index bef16dd74..b5cce3d9e 100644
--- a/src/util/minimal.js
+++ b/src/util/minimal.js
@@ -22,9 +22,6 @@ util.pool = require("@protobufjs/pool");
 // utility to work with the low and high bits of a 64 bit value
 util.LongBits = require("./longbits");
 
-// error subclass indicating a protocol specifc error
-util.ProtocolError = require("./protocolerror");
-
 /**
  * An immuable empty array.
  * @memberof util
@@ -181,17 +178,20 @@ util.longFromHash = function longFromHash(hash, unsigned) {
 
 /**
  * Merges the properties of the source object into the destination object.
+ * @memberof util
  * @param {Object.<string,*>} dst Destination object
  * @param {Object.<string,*>} src Source object
  * @param {boolean} [ifNotSet=false] Merges only if the key is not already set
  * @returns {Object.<string,*>} Destination object
  */
-util.merge = function merge(dst, src, ifNotSet) { // used by converters
+function merge(dst, src, ifNotSet) { // used by converters
     for (var keys = Object.keys(src), i = 0; i < keys.length; ++i)
         if (dst[keys[i]] === undefined || !ifNotSet)
             dst[keys[i]] = src[keys[i]];
     return dst;
-};
+}
+
+util.merge = merge;
 
 /**
  * Converts the first character of a string to lower case.
@@ -202,6 +202,61 @@ util.lcFirst = function lcFirst(str) {
     return str.charAt(0).toLowerCase() + str.substring(1);
 };
 
+/**
+ * Creates a custom error constructor.
+ * @memberof util
+ * @param {string} name Error name
+ * @returns {function} Custom error constructor
+ */
+function newError(name) {
+
+    function CustomError(message, properties) {
+
+        if (!(this instanceof CustomError))
+            return new CustomError(message, properties);
+
+        Error.call(this, message);
+        Object.defineProperty(this, "message", { get: function() { return message; } });
+        if (Error.captureStackTrace)
+            Error.captureStackTrace(this, CustomError);
+        else
+            Object.defineProperty(this, "stack", { value: (new Error()).stack || "" });
+
+        if (properties)
+            merge(this, properties);
+    }
+
+    (CustomError.prototype = Object.create(Error.prototype)).constructor = CustomError;
+
+    Object.defineProperty(CustomError.prototype, "name", { get: function() { return name; } });
+
+    CustomError.prototype.toString = function toString() {
+        return this.name + ": " + this.message;
+    };
+
+    return CustomError;
+}
+
+util.newError = newError;
+
+/**
+ * Constructs a new protocol error.
+ * @classdesc Error subclass indicating a protocol specifc error.
+ * @memberof util
+ * @extends Error
+ * @constructor
+ * @param {string} message Error message
+ * @param {Object.<string,*>=} properties Additional properties
+ * @example
+ * try {
+ *     MyMessage.decode(someBuffer); // throws if required fields are missing
+ * } catch (e) {
+ *     if (e instanceof ProtocolError && e.instance)
+ *         console.log("decoded so far: " + JSON.stringify(e.instance));
+ * }
+ */
+util.ProtocolError = newError("ProtocolError");
+
 /**
  * Builds a getter for a oneof's present field name.
  * @param {string[]} fieldNames Field names
diff --git a/src/util/protocolerror.js b/src/util/protocolerror.js
deleted file mode 100644
index c967489f9..000000000
--- a/src/util/protocolerror.js
+++ /dev/null
@@ -1,51 +0,0 @@
-"use strict";
-module.exports = ProtocolError;
-
-// extends Error
-(ProtocolError.prototype = Object.create(Error.prototype)).constructor = Error;
-
-/**
- * Constructs a new protocol error.
- * @classdesc Error subclass indicating a protocol specifc error.
- * @memberof util
- * @extends Error
- * @constructor
- * @param {string} message Error message
- * @param {Message=} instance So far decoded message instance, if applicable
- * @example
- * try {
- *     MyMessage.decode(someBuffer); // throws if required fields are missing
- * } catch (e) {
- *     if (e instanceof ProtocolError && e.instance)
- *         console.log("decoded so far: " + JSON.stringify(e.instance));
- * }
- */
-function ProtocolError(message, instance) {
-
-    if (!(this instanceof ProtocolError))
-        return new ProtocolError(message, instance);
-
-    /**
-     * Error message.
-     * @type {string}
-     */
-    this.message = message;
-
-    /**
-     * Stack trace.
-     * @type {string}
-     */
-    this.stack = Error(message).stack || /* istanbul ignore next */ ""; // not supported in IE9/Safari5
-
-    /**
-     * So far decoded message instance, if applicable.
-     * @type {?Message}
-     */
-    this.instance = instance || null;
-}
-
-/**
- * Error name (ProtocolError).
- * @type {string}
- */
-ProtocolError.prototype.name = "ProtocolError";