diff --git a/allel/stats/hw.py b/allel/stats/hw.py
index 1ef7e20f..1212584e 100644
--- a/allel/stats/hw.py
+++ b/allel/stats/hw.py
@@ -93,7 +93,8 @@ def heterozygosity_expected(af, ploidy, fill=np.nan):
     out = 1 - np.sum(np.power(af, ploidy), axis=1)
 
     # fill values where allele frequencies could not be calculated
-    af_sum = np.sum(af, axis=1)
+    precision = np.finfo(af.dtype).precision
+    af_sum = np.round(np.sum(af, axis=1), precision)
     with ignore_invalid():
         out[(af_sum < 1) | np.isnan(af_sum)] = fill
 
diff --git a/allel/test/test_stats.py b/allel/test/test_stats.py
index b2ee4b6e..2825e28a 100644
--- a/allel/test/test_stats.py
+++ b/allel/test/test_stats.py
@@ -425,7 +425,10 @@ def refimpl(f, ploidy, fill=0):
             """Limited reference implementation for testing purposes."""
 
             # check allele frequencies sum to 1
-            af_sum = np.sum(f, axis=1)
+            # round to suitable precision for the dtype
+            af_sum = (np.sum(f, axis=1))
+            precision = np.finfo(af_sum.dtype).precision
+            af_sum = np.round(np.sum(f, axis=1), precision)
 
             # assume three alleles
             p = f[:, 0]