diff --git a/python/tests/beagle_numba.py b/python/tests/beagle_numba.py
index f01252acbc..ca3a3b728d 100644
--- a/python/tests/beagle_numba.py
+++ b/python/tests/beagle_numba.py
@@ -50,7 +50,8 @@
 __VERSION__ = "0.0.0"
 __DATE__ = "20XXXXXX"
 
-_ACGT_ALLELES = [0, 1, 2, 3, tskit.MISSING_DATA]
+_ACGT_ALLELES_INT = [0, 1, 2, 3, tskit.MISSING_DATA]
+_ACGT_ALLELES_STR = "ACGT"
 
 
 @dataclass(frozen=True)
@@ -119,16 +120,16 @@ def __post_init__(self):
         assert (
             self.alleles.shape == self.allele_probs.shape
         ), "Dimensions in alleles and allele probabilities don't match."
-        for i in range(len(self.alleles)):
+        for i in range(self.alleles.shape[1]):
             assert np.all(np.isin(self.alleles[:, i], [self.refs[i], self.alts[i]]))
         assert np.all(
-            np.isin(np.unique(self.refs), _ACGT_ALLELES)
+            np.isin(np.unique(self.refs), _ACGT_ALLELES_INT)
         ), "Unrecognized alleles are in REF alleles."
         assert np.all(
-            np.isin(np.unique(self.alts), _ACGT_ALLELES)
+            np.isin(np.unique(self.alts), _ACGT_ALLELES_INT)
         ), "Unrecognized alleles are in ALT alleles."
         assert np.all(
-            np.isin(np.unique(self.alleles), _ACGT_ALLELES)
+            np.isin(np.unique(self.alleles), _ACGT_ALLELES_INT)
         ), "Unrecognized alleles are in alleles."
 
     @property
@@ -170,13 +171,12 @@ def remap_alleles(a):
     :return: Recoded alleles.
     :rtype: np.ndarray(dtype=np.int8)
     """
-    _ALLELES_ACGT = "ACGT"
     b = np.zeros(len(a), dtype=np.int8) - 1  # Encoded as missing by default
     for i in range(len(a)):
         if a[i] in [None, ""]:
             continue
-        elif a[i] in _ALLELES_ACGT:
-            b[i] = _ALLELES_ACGT.index(a[i])
+        elif a[i] in _ACGT_ALLELES_STR:
+            b[i] = _ACGT_ALLELES_STR.index(a[i])
         else:
             raise AssertionError(f"Allele {a[i]} is not recognised.")
     return b
@@ -864,8 +864,8 @@ def write_vcf(ref_ts, impdata, out_file, chr_name="1"):
             line_str = chr_name + "\t"
             line_str += str(int(impdata.site_pos[i])) + "\t"
             line_str += str(i) + "\t"
-            REF = _ACGT_ALLELES[impdata.get_ref_allele_at_site(i)]
-            ALT = _ACGT_ALLELES[impdata.get_alt_allele_at_site(i)]
+            REF = _ACGT_ALLELES_STR[impdata.get_ref_allele_at_site(i)]
+            ALT = _ACGT_ALLELES_STR[impdata.get_alt_allele_at_site(i)]
             line_str += REF + "\t"
             line_str += ALT + "\t"
             # QUAL