diff --git a/CHANGELOG.md b/CHANGELOG.md
index ae3e5321d..0614b78f4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,7 @@ All notable changes to this project will be documented in this file.
 ### Removed
 ### Fixed
 - [#1713](https://github.com/pints-team/pints/pull/1713) Fixed Numpy 2.4.1 compatibility issues.
+- [#1690][https://github.com/pints-team/pints/pull/1690) Fixed bug in optimisation controller if population size left at `None`.
 
 
 ## [0.5.1] - 2025-09-26
diff --git a/pints/_optimisers/__init__.py b/pints/_optimisers/__init__.py
index 17c99d86a..55bd9a45a 100644
--- a/pints/_optimisers/__init__.py
+++ b/pints/_optimisers/__init__.py
@@ -622,6 +622,10 @@ def run(self):
             pop_size = 1
             if isinstance(self._optimiser, PopulationBasedOptimiser):
                 pop_size = self._optimiser.population_size()
+                if pop_size is None:
+                    pop_size = self._optimiser.suggested_population_size(
+                        n_workers if self._parallel else None)
+                    self._optimiser.set_population_size(pop_size)
                 if self._log_to_screen:
                     print('Population size: ' + str(pop_size))
 
diff --git a/pints/tests/test_opt_controller.py b/pints/tests/test_opt_controller.py
index 2313039bb..ca5e10c2e 100755
--- a/pints/tests/test_opt_controller.py
+++ b/pints/tests/test_opt_controller.py
@@ -364,31 +364,17 @@ def test_stopping_no_criterion(self):
         opt.set_max_unchanged_iterations(None)
         self.assertRaises(ValueError, opt.run)
 
-    def test_set_population_size(self):
-        # Tests the set_population_size method for this optimiser.
+    def test_population_size_not_set(self):
+        # Population size can be None: then suggested should be used
 
-        r = pints.toy.RosenbrockError()
-        x = np.array([1.01, 1.01])
-        opt = pints.OptimisationController(r, x, method=method)
-        m = opt.optimiser()
-        n = m.population_size()
-        m.set_population_size(n + 1)
-        self.assertEqual(m.population_size(), n + 1)
-
-        # Test invalid size
-        self.assertRaisesRegex(
-            ValueError, 'at least 1', m.set_population_size, 0)
-
-        # test hyper parameter interface
-        self.assertEqual(m.n_hyper_parameters(), 1)
-        m.set_hyper_parameters([n + 2])
-        self.assertEqual(m.population_size(), n + 2)
-        self.assertRaisesRegex(
-            ValueError, 'at least 1', m.set_hyper_parameters, [0])
-
-        # Test changing during run
-        m.ask()
-        self.assertRaises(Exception, m.set_population_size, 2)
+        model = pints.toy.ParabolicError()
+        opt = pints.OptimisationController(model, [1, 1], method=pints.CMAES)
+        opt.optimiser().set_population_size(None)
+        opt.set_log_to_screen(True)
+        opt.set_max_iterations(3)
+        with StreamCapture() as c:
+            opt.run()
+        self.assertIn('Population size: 6', c.text())
 
     def test_parallel(self):
         # Test parallelised running.
diff --git a/pints/tests/test_opt_population_based.py b/pints/tests/test_opt_population_based.py
new file mode 100755
index 000000000..a5c6d0caf
--- /dev/null
+++ b/pints/tests/test_opt_population_based.py
@@ -0,0 +1,53 @@
+#!/usr/bin/env python3
+#
+# Tests the shared methods of the PopulationBasedOptimiser
+#
+# This file is part of PINTS (https://github.com/pints-team/pints/) which is
+# released under the BSD 3-clause license. See accompanying LICENSE.md for
+# copyright notice and full license details.
+#
+import unittest
+
+import numpy as np
+
+import pints
+import pints.toy
+
+
+class TestPopulationBasedOptimiser(unittest.TestCase):
+    """
+    Tests the  shared methods of the PopulationBasedOptimiser.
+    """
+    def setUp(self):
+        """ Called before every test """
+        np.random.seed(1)
+
+    def test_population_size(self):
+
+        r = pints.toy.RosenbrockError()
+        x = np.array([1.01, 1.01])
+        opt = pints.OptimisationController(r, x, method=pints.XNES)
+        m = opt.optimiser()
+        n = m.population_size()
+        m.set_population_size(n + 1)
+        self.assertEqual(m.population_size(), n + 1)
+
+        # Test invalid size
+        self.assertRaisesRegex(
+            ValueError, 'at least 1', m.set_population_size, 0)
+
+        # test hyper parameter interface
+        self.assertEqual(m.n_hyper_parameters(), 1)
+        m.set_hyper_parameters([n + 2])
+        self.assertEqual(m.population_size(), n + 2)
+        self.assertRaisesRegex(
+            ValueError, 'at least 1', m.set_hyper_parameters, [0])
+
+        # Test changing during run
+        m.ask()
+        self.assertRaises(Exception, m.set_population_size, 2)
+
+
+if __name__ == '__main__':
+    unittest.main()
+