Difference between Non-Static Method without self and static method?

Question

My unittests kept failing with a quite unspecific error without a line or additional information:

In  testMainInno (main.MainTestCases) :

 TypeError: testMainInno() missing 1 required positional argument: 'dbname'

Fortunately this method is only called a single time:

class MainTests(unittest.TestCase):
    def setUp(self):
        mtc.testMainInno(self.defaultDBName)

Which refers to this class with the following method:

class MainTestCases(unittest.TestCase):    
    @staticmethod
    def testMainInno(dbname):
        #doStuff

I decided to declare testMainInno as a static method as it doesn't require anything from the class and I see no purpose in adding self.

The error is especially weird as it seems the method is executed properly, I can debug it and the pdb goes into it, executing everything. I decided to remove the @staticmethod-Decorator and the error is gone.

I am now a little puzzled as I am used to getting errors when I declare non-static methods that imply a usage of self. And even if it wouldn't, what kind of error is it that doesn't have any effect as everything gets executed as planned?