What causes the "invalid operand types for the binary operator" == "with the second type <nulltype>?

When I check if an instance of StoredCredential is null

if(storedCredential != null){
  //do something
}

Netbeans 7.1 shows the following warning:

bad operand types for binary operator '=='
  first type:  com.blah.dbcore.mypublic.beans.StoredCredential
  second type: <nulltype>

It compiles correctly and does not generate RuntimeExceptions, but Netbeans still shows a warning. Verifying that the instance is not null, with '! = ', Gives the same error.

The following code does the same, but is much less clear:

if (!(storedCredential instanceof StoredCredential)) {
  //do something
}

Netbeans has no problem with this expression.

Since it has no compilation problems, I assume that something is wrong with the way I added the .jar containing the class to the library. This is the only problem I encountered with this jar, but no other class in this .jar gives me this problem.