diff --git a/build-cython.py b/build-cython.py
index ae0044c5..cdce1a2b 100644
--- a/build-cython.py
+++ b/build-cython.py
@@ -15,7 +15,7 @@ def build_cython() -> None:
         include_dirs=[numpy.get_include()],
         extra_compile_args=["-O3", "-march=native", "-Wall"],
     )
-    ext_modules = cythonize(extension)
+    ext_modules = cythonize(extension, compiler_directives={"language_level": "3"})
     distribution = Distribution(dict(ext_modules=ext_modules))
 
     cmd = build_ext(distribution)
diff --git a/qldpc/codes/_distance.pyx b/qldpc/codes/_distance.pyx
index c60d9657..ea09b127 100644
--- a/qldpc/codes/_distance.pyx
+++ b/qldpc/codes/_distance.pyx
@@ -1,5 +1,3 @@
-# cython: language_level=3
-
 # C imports
 from libc.stdint cimport uint64_t
 
@@ -24,8 +22,8 @@ cimport numpy as cnp
 # utility functions
 
 
-cdef uint64_t ODD_BITS_MASK = 0x5555555555555555
-cdef uint64_t EVEN_BITS_MASK = 0xAAAAAAAAAAAAAAAA
+cdef uint64_t ODD_BITS_MASK = 0x5555555555555555  # 1 on the odd bits of a 64-bit integer
+cdef uint64_t EVEN_BITS_MASK = 0xAAAAAAAAAAAAAAAA  # 1 on the even bits of a 64-bit integer
 
 
 cdef uint64_t hamming_weight(uint64_t num):