diff --git a/dask_ml/model_selection/_split.py b/dask_ml/model_selection/_split.py
index 26134da5b..90e342b19 100644
--- a/dask_ml/model_selection/_split.py
+++ b/dask_ml/model_selection/_split.py
@@ -424,9 +424,9 @@ def train_test_split(
         test_size = 0.1
 
     if train_size is None and test_size is not None:
-        train_size = 1 - test_size
+        train_size = round(1 - test_size, 6)
     if test_size is None and train_size is not None:
-        test_size = 1 - train_size
+        test_size = round(1 - train_size, 6)
 
     if options:
         raise TypeError("Unexpected options {}".format(options))