diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index ec99e69c..b342f85e 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -14,7 +14,7 @@ repos:
   rev: 19.3b0
   hooks:
   - id: black
-    language_version: python3.7
+    language_version: python3
     exclude: lib
 
 - repo: https://github.com/dfm/black_nbconvert
diff --git a/starry/_core/ops/lib/include/filter.h b/starry/_core/ops/lib/include/filter.h
index 8096414e..9ee7cd67 100644
--- a/starry/_core/ops/lib/include/filter.h
+++ b/starry/_core/ops/lib/include/filter.h
@@ -21,7 +21,7 @@ Filter operations on a spherical harmonic map.
 */
 template <typename Scalar> class Filter {
 protected:
-  basis::Basis<Scalar> &B;
+  basis::Basis<Scalar> B;
   const int ydeg; /**< */
   const int Ny;   /**< Number of spherical harmonic `(l, m)` coefficients */
   const int udeg; /**< */