diff --git a/lib/session.js b/lib/session.js
index a724979..56d28b5 100644
--- a/lib/session.js
+++ b/lib/session.js
@@ -295,7 +295,7 @@ console.log(retPduObj);
 
 			if (smsData.smsId !== undefined) {
 				// Todo: Generate random ID
-				smsData.smsId = 666;
+				smsData.smsId = '666';
 			}
 
 			smsObj.smsId = smsData.smsId;
diff --git a/lib/utils.js b/lib/utils.js
index a8b0bbc..f1cf7b2 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -3,6 +3,140 @@
 var log  = require('winston'),
     defs = require('./defs');
 
+/**
+ * Calcualte cmdLength from object
+ *
+ * @param obj obj
+ * @param func callback(err, cmdLength)
+ */
+function calcCmdLength(obj, callback) {
+	var cmdLength = 16, // All commands are at least 16 octets long
+	    err,
+	    param,
+	    paramType,
+	    tlvValue,
+	    tlvName,
+	    tlvDef;
+
+	// Handle params - All command params should always exists, even if they do not contain data.
+	for (param in defs.cmds[obj.cmdName].params) {
+
+		// Get the parameter type, int, string, cstring etc.
+		// This is needed so we can calculate length etc
+		paramType = defs.cmds[obj.cmdName].params[param].type;
+
+		if (obj.params[param] === undefined) {
+			obj.params[param] = paramType.default;
+		}
+
+		if (isNaN(paramType.size(obj.params[param]))) {
+			err = new Error('Invalid param value "' + obj.params[param] + '" for param "' + param + '" and command "' + obj.cmdName + '". Is it of the right type?');
+			log.error('larvitsmpp: lib/utils.js: calcCmdLength() - ' + err.message);
+			callback(err);
+			return;
+		}
+
+		cmdLength += paramType.size(obj.params[param]);
+	}
+
+	// TLV params - optional parameters
+	for (tlvName in obj.tlvs) {
+		tlvValue = obj.tlvs[tlvName].tagValue;
+		tlvDef   = defs.tlvsById[obj.tlvs[tlvName].tagId];
+
+		if (tlvDef === undefined) {
+			tlvDef = defs.tlvs.default;
+		}
+
+		cmdLength += tlvDef.type.size(tlvValue) + 4;
+	}
+
+	callback(null, cmdLength);
+}
+
+/**
+ * Write PDU to buffer
+ *
+ * @param obj obj - the PDU object to be written to buffer
+ * @param int cmdLength - The length of the pdu buffer
+ * @param func callback(err, buff)
+ */
+function writeBuffer(obj, cmdLength, callback) {
+	var offset = 16, // Start the offset on the body
+	    buff,
+	    param,
+	    paramType,
+	    tlvId,
+	    tlvName,
+	    tlvValue,
+	    tlvDef,
+	    tlvSize,
+	    err;
+
+	if (isNaN(cmdLength) || cmdLength < 16) {
+		if (isNaN(cmdLength)) {
+			err = new Error('cmdLength is NaN');
+		}
+
+		if (cmdLength < 16) {
+			err = new Error('cmdLength is less than 16 (' + cmdLength + ')');
+		}
+
+		log.error('larvitsmpp: lib/utils.js: objToPdu() - writeBuffer() - ' + err.message);
+		callback(err);
+		return;
+	}
+
+	buff = new Buffer(cmdLength);
+
+	// Write PDU header
+	try {
+		buff.writeUInt32BE(cmdLength, 0);                  // Command length for the first 4 octets
+		buff.writeUInt32BE(defs.cmds[obj.cmdName].id, 4);  // Command id for the second 4 octets
+		buff.writeUInt32BE(defs.errors[obj.cmdStatus], 8); // Command status for the third 4 octets
+		buff.writeUInt32BE(obj.seqNr, 12);                 // Sequence number as the fourth 4 octets
+	} catch (e) {
+		err = new Error('Could not write PDU header, catched err: ' + e.message, obj);
+		log.error('larvitsmpp: lib/utils.js: writeBuffer() - ' + err.message);
+		callback(err);
+		return;
+	}
+
+	// Cycle through the defs list to make sure the params are in the right order
+	for (param in defs.cmds[obj.cmdName].params) {
+		paramType = defs.cmds[obj.cmdName].params[param].type;
+
+		// Write parameter value to buffer using the types method write()
+		paramType.write(obj.params[param], buff, offset);
+
+		// Increase the offset for the next param
+		offset += paramType.size(obj.params[param]);
+	}
+
+	// Cycle through the tlvs
+	for (tlvName in obj.tlvs) {
+		tlvId    = obj.tlvs[tlvName].tagId;
+		tlvValue = obj.tlvs[tlvName].tagValue;
+		tlvDef   = defs.tlvsById[tlvId];
+
+		if (tlvDef === undefined) {
+			tlvDef = defs.tlvs.default;
+		}
+
+		tlvSize  = tlvDef.type.size(tlvValue);
+
+		log.silly('larvitsmpp: lib/utils.js: objToPdu() - writeBuffer() - Writing TLV "' + tlvName + '" offset: ' + offset + ' value: "' + tlvValue + '"');
+
+		buff.writeUInt16BE(tlvId, offset);
+		buff.writeUInt16BE(tlvSize, offset + 2);
+		tlvDef.type.write(tlvValue, buff, offset + 4);
+
+		offset += tlvDef.type.size(tlvValue) + 4;
+	}
+
+	callback(null, buff);
+}
+
 /**
  * Decode a short_message
  *
@@ -175,100 +309,9 @@ function pduToObj(pdu, callback) {
  * @param func callback(err, pdu)
  */
 function objToPdu(obj, callback) {
-	var cmdLength = 16, // All commands are at least 16 octets long
-	    err       = null,
+	var err = null,
 	    shortMsg,
-	    param,
-	    paramType,
-	    buff,
-	    seqNr,
-	    tlvId,
-	    tlvName,
-	    tlvValue,
-	    tlvDef,
-	    tlvSize;
-
-	function calcCmdLength() {
-		// Handle params - All command params should always exists, even if they do not contain data.
-		for (param in defs.cmds[obj.cmdName].params) {
-
-			// Get the parameter type, int, string, cstring etc.
-			// This is needed so we can calculate length etc
-			paramType = defs.cmds[obj.cmdName].params[param].type;
-
-			if (obj.params[param] === undefined) {
-				obj.params[param] = paramType.default;
-			}
-
-			cmdLength += paramType.size(obj.params[param]);
-		}
-
-		// TLV params - optional parameters
-		for (tlvName in obj.tlvs) {
-			tlvValue = obj.tlvs[tlvName].tagValue;
-			tlvDef   = defs.tlvsById[obj.tlvs[tlvName].tagId];
-
-			if (tlvDef === undefined) {
-				tlvDef = defs.tlvs.default;
-			}
-
-			cmdLength += tlvDef.type.size(tlvValue) + 4;
-		}
-	}
-
-	// Used to write buffer once the command length is known
-	function writeBuffer() {
-		var offset = 16, // Start the offset on the body
-		    err;
-
-
-		buff = new Buffer(cmdLength);
-
-		// Write PDU header
-		try {
-			buff.writeUInt32BE(cmdLength, 0);                  // Command length for the first 4 octets
-			buff.writeUInt32BE(defs.cmds[obj.cmdName].id, 4);  // Command id for the second 4 octets
-			buff.writeUInt32BE(defs.errors[obj.cmdStatus], 8); // Command status for the third 4 octets
-			buff.writeUInt32BE(seqNr, 12);                     // Sequence number as the fourth 4 octets
-		} catch (e) {
-			err = new Error('Could not write PDU header, catched err: ' + e.message, obj);
-			log.error('larvitsmpp: lib/utils.js: objToPdu() - writeBuffer() - ' + err.message);
-			callback(err);
-			return;
-		}
-
-		// Cycle through the defs list to make sure the params are in the right order
-		for (param in defs.cmds[obj.cmdName].params) {
-			paramType = defs.cmds[obj.cmdName].params[param].type;
-
-			// Write parameter value to buffer using the types method write()
-			paramType.write(obj.params[param], buff, offset);
-
-			// Increase the offset for the next param
-			offset += paramType.size(obj.params[param]);
-		}
-
-		// Cycle through the tlvs
-		for (tlvName in obj.tlvs) {
-			tlvId    = obj.tlvs[tlvName].tagId;
-			tlvValue = obj.tlvs[tlvName].tagValue;
-			tlvDef   = defs.tlvsById[tlvId];
-
-			if (tlvDef === undefined) {
-				tlvDef = defs.tlvs.default;
-			}
-
-			tlvSize  = tlvDef.type.size(tlvValue);
-
-			log.silly('larvitsmpp: lib/utils.js: objToPdu() - writeBuffer() - Writing TLV "' + tlvName + '" offset: ' + offset + ' value: "' + tlvValue + '"');
-
-			buff.writeUInt16BE(tlvId, offset);
-			buff.writeUInt16BE(tlvSize, offset + 2);
-			tlvDef.type.write(tlvValue, buff, offset + 4);
-
-			offset += tlvDef.type.size(tlvValue) + 4;
-		}
-	}
+	    seqNr;
 
 	// Check so the command is ok
 	if (defs.cmds[obj.cmdName] === undefined) {
@@ -322,11 +365,16 @@ function objToPdu(obj, callback) {
 		log.silly('larvitsmpp: lib/utils.js: objToPdu() - encoding message "' + shortMsg + '" to "' + obj.params.short_message.toString('hex') + '"');
 	}
 
-	calcCmdLength();
-	writeBuffer();
-	if (buff) {
-		callback(null, buff);
-	}
+	calcCmdLength(obj, function(err, cmdLength) {
+		if (err) {
+			callback(err);
+			return;
+		}
+
+		writeBuffer(obj, cmdLength, function(err, buff) {
+			callback(err, buff);
+		});
+	});
 }
 
 /**