diff --git a/fdroidserver/up.py b/fdroidserver/up.py
index f6b561f3..24c71da5 100644
--- a/fdroidserver/up.py
+++ b/fdroidserver/up.py
@@ -148,13 +148,16 @@ def get_virt_cpus_opt(cpus):
     If no CPU count is configured, calculate a reasonable default value.
 
     """
-    if cpus:
-        return cpus
     cpu_cnt = os.cpu_count()
-    if cpu_cnt < 8:
-        return max(1, int(0.5 * cpu_cnt))
-    # use a quarter of available CPUs if there
-    return 2 + int(0.25 * cpu_cnt)
+    if not cpus:
+        if cpu_cnt < 8:
+            cpus = max(1, int(0.5 * cpu_cnt))
+        else:
+            # use a quarter of available CPUs if there
+            cpus = 2 + int(0.25 * cpu_cnt)
+    if min(cpus, cpu_cnt) != cpus:
+        logging.warning(f'Capping {cpus} CPUs to how many are available ({cpu_cnt}).')
+    return min(cpus, cpu_cnt)
 
 
 def get_virt_memory_opt(memory):
diff --git a/tests/test_up.py b/tests/test_up.py
index 09820b73..1e75ed8e 100755
--- a/tests/test_up.py
+++ b/tests/test_up.py
@@ -145,6 +145,16 @@ class Up_run_vagrant(UpTest):
 
 
 class Up_options(UpTest):
+    def test_get_virt_cpus_opt_default(self):
+        self.assertTrue(up.get_virt_cpus_opt(None) > 0)
+
+    def test_get_virt_cpus_opt_too_small(self):
+        self.assertTrue(up.get_virt_cpus_opt(0.1) > 0)
+
+    def test_get_virt_cpus_opt_too_big(self):
+        with self.assertLogs():
+            self.assertEqual(up.get_virt_cpus_opt(99999999), os.cpu_count())
+
     def test_get_virt_memory_opt_default(self):
         self.assertEqual(up.get_virt_memory_opt(None), 6 * 1024**3)