diff --git a/heracles/catalog/base.py b/heracles/catalog/base.py
index bf02d84..e19c5e0 100644
--- a/heracles/catalog/base.py
+++ b/heracles/catalog/base.py
@@ -419,7 +419,7 @@ def _names(self) -> TypeVar("Unknown"):
         ...
 
     @abstractmethod
-    def _size(self, selection: TypeVar("Unknown")) -> TypeVar("Unknown"):
+    def _size(self, selection: TypeVar("Unknown")) -> int:
         """Abstract method to return the size of the catalogue or selection.
 
         Args:
diff --git a/heracles/covariance.py b/heracles/covariance.py
index 6dd8217..8bf7c02 100644
--- a/heracles/covariance.py
+++ b/heracles/covariance.py
@@ -17,7 +17,6 @@
 # You should have received a copy of the GNU Lesser General Public
 # License along with Heracles. If not, see <https://www.gnu.org/licenses/>.
 """Module for covariance matrix computation."""
-
 import logging
 import time
 from datetime import timedelta
@@ -26,6 +25,7 @@
 
 import healpy as hp
 import numpy as np
+import numpy.typing as npt
 
 from ._kmeans_radec import kmeans_sample
 
@@ -37,9 +37,9 @@ class SampleCovariance(np.ndarray):
 
     def __new__(
         cls,
-        nrows: TypeVar("Unknown"),
-        ncols: Optional[TypeVar("Unknown")] = None,
-    ) -> TypeVar("Unknown"):
+        nrows: int,
+        ncols: Optional[int] = None,
+    ) -> "SampleCovariance":
         """_summary_.
 
         Args:
@@ -57,7 +57,7 @@ def __new__(
         cov.sample_col_mean = np.zeros(ncols)
         return cov
 
-    def __array_finalize__(self, cov: TypeVar("Unknown")) -> None:
+    def __array_finalize__(self, cov: Optional["SampleCovariance"]) -> None:
         """_summary_.
 
         Args:
@@ -74,9 +74,9 @@ def __array_finalize__(self, cov: TypeVar("Unknown")) -> None:
 
 
 def add_sample(
-    cov: TypeVar("Unknown"),
-    x: TypeVar("Unknown"),
-    y: Optional[TypeVar("Unknown")] = None,
+    cov: "SampleCovariance",
+    x: npt.NDArray,
+    y: Optional[npt.NDArray] = None,
 ) -> None:
     """Add a sample to a sample covariance matrix.
 
@@ -106,7 +106,7 @@ def add_sample(
         cov += (np.outer(delta, y - cov.sample_col_mean) - cov) / (cov.sample_count - 1)
 
 
-def update_covariance(cov: TypeVar("Unknown"), sample: TypeVar("Unknown")) -> None:
+def update_covariance(cov: "SampleCovariance", sample: TypeVar("Unknown")) -> None:
     """Update a set of sample covariances given a sample.
 
     Args:
@@ -139,7 +139,7 @@ def update_covariance(cov: TypeVar("Unknown"), sample: TypeVar("Unknown")) -> No
 
 def jackknife_regions_kmeans(
     fpmap: TypeVar("Unknown"),
-    n: TypeVar("Unknown"),
+    n: int,
     *,
     maxrepeat: int = 5,
     maxiter: int = 1_000,
diff --git a/heracles/util.py b/heracles/util.py
index d8715e0..3b61784 100644
--- a/heracles/util.py
+++ b/heracles/util.py
@@ -43,7 +43,7 @@ def __init__(self, out: io.IOBase = sys.stdout) -> None:
     def start(
         self,
         total: TypeVar("Unknown"),
-        title: Optional[TypeVar("Unknown")] = None,
+        title: Optional[str] = None,
     ) -> None:
         """Start new progress.