diff --git a/plotnine/stats/stat_summary_bin.py b/plotnine/stats/stat_summary_bin.py
index 5e92c269e8..47585eb944 100644
--- a/plotnine/stats/stat_summary_bin.py
+++ b/plotnine/stats/stat_summary_bin.py
@@ -1,4 +1,5 @@
 from typing import TYPE_CHECKING, cast
+from warnings import warn
 
 import numpy as np
 import pandas as pd
@@ -107,8 +108,9 @@ class stat_summary_bin(stat):
     def setup_params(self, data):
         keys = ("fun_data", "fun_y", "fun_ymin", "fun_ymax")
         if not any(self.params[k] for k in keys):
-            PlotnineWarning(
-                "No summary function, supplied, defaulting to mean_se()"
+            warn(
+                "No summary function supplied, defaulting to mean_se()",
+                PlotnineWarning,
             )
             self.params["fun_data"] = "mean_se"
 
diff --git a/tests/test_stat_summary_bin.py b/tests/test_stat_summary_bin.py
index a80e63be9e..5f0dfd45c9 100644
--- a/tests/test_stat_summary_bin.py
+++ b/tests/test_stat_summary_bin.py
@@ -1,7 +1,9 @@
 import numpy as np
 import pandas as pd
+import pytest
 
 from plotnine import aes, ggplot, stat_summary_bin
+from plotnine.exceptions import PlotnineWarning
 
 data = pd.DataFrame(
     {
@@ -31,3 +33,11 @@ def test_continuous_x():
 def test_setting_binwidth():
     p = ggplot(data, aes("xc", "y")) + stat_summary_bin(binwidth=3, geom="bar")
     assert p == "setting_binwidth"
+
+
+def test_no_summary_function_warns():
+    # A missing summary function falls back to mean_se(), and the
+    # fallback must be announced.
+    p = ggplot(data, aes("xc", "y")) + stat_summary_bin(bins=5)
+    with pytest.warns(PlotnineWarning, match="defaulting to mean_se"):
+        p._build()