diff --git a/snakemake/workflow.py b/snakemake/workflow.py
index 97296c46d..c89d69033 100644
--- a/snakemake/workflow.py
+++ b/snakemake/workflow.py
@@ -165,7 +165,9 @@ def __init__(
         self.global_resources["_cores"] = cores
         self.global_resources["_nodes"] = nodes
 
-        self.rerun_triggers = frozenset(rerun_triggers)
+        self.rerun_triggers = (
+            frozenset(rerun_triggers) if rerun_triggers is not None else frozenset()
+        )
         self._rules = OrderedDict()
         self.default_target = None
         self._workdir = None
diff --git a/tests/testapi.py b/tests/testapi.py
index 12400bc8d..ef6e2d13a 100644
--- a/tests/testapi.py
+++ b/tests/testapi.py
@@ -17,6 +17,28 @@ def test_keep_logger():
         snakemake(path, workdir=tmpdir, keep_logger=True)
 
 
+def test_workflow_calling():
+    with tempfile.TemporaryDirectory() as tmpdir:
+        path = os.path.join(tmpdir, "Snakefile")
+        with open(path, "w") as f:
+            print(
+                dedent(
+                    """
+                rule:
+                    output: 'result.txt'
+                    run:
+                        with open(output[0], 'w') as f:
+                            print("hello", file=f)
+                """
+                ),
+                file=f,
+            )
+        workflow = Workflow(
+            snakefile=snakefile,
+            overwrite_workdir=tmpdir,
+        )
+
+
 def test_run_script_directive():
     with tempfile.TemporaryDirectory() as tmpdir:
         path = os.path.join(tmpdir, "Snakefile")