MySQL ErrorNo: 150

Hi, i dont know what im doing wrong here. i always have the same error:

ERROR 1500 (HY0000): Cant Create table errno: 150 (for Foreign Keys)

can someone help me out? thanks.

here are my table statements:

create table campaigns(campaignname varchar(20) PRIMARY KEY,
companyname varchar(20),
teamcolor varchar(15),
priority varchar(20),
datereceived date,
datedue date,
status varchar(20),
projectmanager varchar(30));

create table builds(campaignname varchar(20),
buildname varchar(15),
adtype varchar(15),
complexity varchar(10),
category varchar(15),
FOREIGN KEY (campaignname) REFERENCES campaigns(campaignname) ON DELETE CASCADE,
PRIMARY KEY(campaignname,buildname));

create table tasks(campaignname varchar(20),
buildname varchar(15),
taskname varchar(15),
taskdetails varchar(50),
assignedto varchar(20),
dependenton varchar(15),
comments varchar(50),
lastupdate datetime,
PRIMARY KEY(taskname, buildname),
FOREIGN KEY (buildname) REFERENCES builds(buildname) ON DELETE CASCADE);

I get problems with the tasks table... what am i doing wrong here?