diff --git a/filterpy/kalman/kalman_filter.py b/filterpy/kalman/kalman_filter.py
index 63504ae..c4d411e 100644
--- a/filterpy/kalman/kalman_filter.py
+++ b/filterpy/kalman/kalman_filter.py
@@ -321,7 +321,7 @@ def test_matrix_dimensions(self, z=None, H=None, R=None, F=None, Q=None):
                "Shape of P must be ({},{}), but is {}".format(
                self.dim_x, self.dim_x, P.shape)
 
-        assert self._F.shape == (self.dim_x, self.dim_x), \
+        assert F.shape == (self.dim_x, self.dim_x), \
                "Shape of F must be ({},{}), but is {}".format(
                self.dim_x, self.dim_x, F.shape)