Error when passing pointer to imported Extension type method while having no problem with the same class without import

IMPORTANT UPDATE: reproducible example here

I have code similar to one in documentation, so that the class is created from pointer. It works perfectly, when both class definition and class instance creation are in the same file.

main.pyx:

cdef some_type * ptr_to_wrapped_type = <some_type *>malloc(sizeof(some_type))

#next part from documentation
cdef class WrapperClass:
    """A wrapper class for a C/C++ data structure"""
    cdef my_c_struct *_ptr
    cdef bint ptr_owner

    @staticmethod
    cdef WrapperClass from_ptr(my_c_struct *_ptr, bint owner=False):
        """Factory function to create WrapperClass objects from
        given my_c_struct pointer.
        Setting ``owner`` flag to ``True`` causes
        the extension type to ``free`` the structure pointed to by ``_ptr``
        when the wrapper object is deallocated."""
        # Call to __new__ bypasses __init__ constructor
        cdef WrapperClass wrapper = WrapperClass.__new__(WrapperClass)
        wrapper._ptr = _ptr
        wrapper.ptr_owner = owner
        return wrapper
...

data = WrapperClass.from_ptr(ptr_to_wrapped_type, owner=True)

What I want is to move class definition into separate file and import it in main. This is also described in docs and that's what I have:

wrapper.pxd:

# Example C struct
ctypedef struct my_c_struct:
    int a
    int b

cdef class WrapperClass:
    """A wrapper class for a C/C++ data structure"""
    cdef my_c_struct *_ptr
    cdef bint ptr_owner

wrapper.pyx(class definition from docs):

from libc.stdlib cimport malloc, free

cdef class WrapperClass:
    """A wrapper class for a C/C++ data structure"""
    cdef my_c_struct *_ptr
    cdef bint ptr_owner

    def __cinit__(self):
        self.ptr_owner = False

    def __dealloc__(self):
        # De-allocate if not null and flag is set
        if self._ptr is not NULL and self.ptr_owner is True:
            free(self._ptr)
            self._ptr = NULL

    @staticmethod
    cdef WrapperClass from_ptr(my_c_struct *_ptr, bint owner):
        """Factory function to create WrapperClass objects from
        given my_c_struct pointer.
        Setting ``owner`` flag to ``True`` causes
        the extension type to ``free`` the structure pointed to by ``_ptr``
        when the wrapper object is deallocated."""
        # Call to __new__ bypasses __init__ constructor
        cdef WrapperClass wrapper = WrapperClass.__new__(WrapperClass)
        wrapper._ptr = _ptr
        wrapper.ptr_owner = owner
        return wrapper
...

And finally in main.pyx:

cdef some_type * ptr_to_wrapped_type = <some_type *>malloc(sizeof(some_type))

from wrapper cimport WrapperClass

data = WrapperClass.from_ptr(ptr_to_wrapped_type, owner=True)

Unfortunately, at compilation I get an Error:

data = WrapperClass.from_ptr(ptr_to_wrapped_type, owner=True)
                            ^
------------------------------------------------------------

Cannot convert 'some_type *' to Python object

What is wrong?