diff --git a/lib/imap.js b/lib/imap.js
index 8f842ab..3b236b2 100644
--- a/lib/imap.js
+++ b/lib/imap.js
@@ -28,6 +28,7 @@ var CRLF = '\r\n',
     RE_TEXT_CODE = /([^ ]+)(?: (.*))?$/,
     RE_RES_IDLE = /^IDLE /i,
     RE_RES_NOOP = /^NOOP /i,
+    RE_PARTID = /^(?:[\d]+[\.]{0,1})*[\d]+$/,
     //RE_ISPARTIAL = /<(\d+)>$/,
     RE_DBLQ = /"/g,
     RE_CMD = /^([^ ]+)(?: |$)/,
@@ -1045,9 +1046,9 @@ ImapConnection.prototype._fetch = function(which, uids, options, what, cb) {
   for (var i = 0, wp, pprefix, len = what.length; i < len; ++i) {
     wp = what[i];
     parse = true;
-    if (wp.id !== undefined && !/^(?:[\d]+[\.]{0,1})*[\d]+$/.test(''+wp.id))
+    if (wp.id !== undefined && !RE_PARTID.test(''+wp.id))
       throw new Error('Invalid part id: ' + wp.id);
-    if (( (wp.headers
+    if (( (typeof wp.headers === 'object'
            && (!wp.headers.fields
                || (Array.isArray(wp.headers.fields)
                    && wp.headers.fields.length === 0)
@@ -1055,7 +1056,7 @@ ImapConnection.prototype._fetch = function(which, uids, options, what, cb) {
            && wp.headers.parse === false
           )
           ||
-          (wp.headersNot
+          (typeof wp.headersNot === 'object'
            && (!wp.headersNot.fields
                || (Array.isArray(wp.headersNot.fields)
                    && wp.headersNot.fields.length === 0)