diff --git a/cobra/preprocessing/preprocessor.py b/cobra/preprocessing/preprocessor.py
index 7e23d67..3cd94c8 100644
--- a/cobra/preprocessing/preprocessor.py
+++ b/cobra/preprocessing/preprocessor.py
@@ -14,6 +14,7 @@
 import inspect
 from datetime import datetime
 import time
+import math
 import logging
 from random import shuffle
 
@@ -361,9 +362,9 @@ def train_selection_validation_split(data: pd.DataFrame,
         pd.DataFrame
             DataFrame with additional split column
         """
-        if train_prop + selection_prop + validation_prop != 1.0:
+        if not math.isclose(train_prop + selection_prop + validation_prop, 1.0):
             raise ValueError("The sum of train_prop, selection_prop and "
-                             "validation_prop cannot differ from 1.0")
+                             "validation_prop must be 1.0.")
 
         if train_prop == 0.0:
             raise ValueError("train_prop cannot be zero!")
diff --git a/tests/preprocessing/test_preprocessor.py b/tests/preprocessing/test_preprocessor.py
index c2e4fcd..80f6d73 100644
--- a/tests/preprocessing/test_preprocessor.py
+++ b/tests/preprocessing/test_preprocessor.py
@@ -16,13 +16,19 @@ def does_not_raise():
 
 class TestPreProcessor:
 
-    @pytest.mark.parametrize(("train_prop, selection_prop, "
-                              "validation_prop, expected_sizes"),
+    @pytest.mark.parametrize("train_prop, selection_prop, validation_prop, "
+                             "expected_sizes",
                              [(0.6, 0.2, 0.2, {"train": 6,
                                                "selection": 2,
                                                "validation": 2}),
                               (0.7, 0.3, 0.0, {"train": 7,
-                                               "selection": 3})])
+                                               "selection": 3}),
+                              # Error "The sum of train_prop, selection_prop and
+                              # validation_prop must be 1.0." should not be
+                              # raised:
+                              (0.7, 0.2, 0.1, {"train": 7,
+                                               "selection": 2,
+                                               "validation": 1})])
     def test_train_selection_validation_split(self, train_prop: float,
                                               selection_prop: float,
                                               validation_prop: float,
@@ -50,7 +56,7 @@ def test_train_selection_validation_split(self, train_prop: float,
     def test_train_selection_validation_split_error_wrong_prop(self):
 
         error_msg = ("The sum of train_prop, selection_prop and "
-                     "validation_prop cannot differ from 1.0")
+                     "validation_prop must be 1.0.")
         train_prop = 0.7
         selection_prop = 0.3