diff --git a/src/wuttaweb/handler.py b/src/wuttaweb/handler.py
index 3b63022..615eae4 100644
--- a/src/wuttaweb/handler.py
+++ b/src/wuttaweb/handler.py
@@ -70,7 +70,15 @@ class WebHandler(GenericHandler):
         url = self.config.get("wuttaweb.favicon_url")
         if url:
             return url
-        return self.get_fanstatic_url(request, static.favicon)
+
+        # nb. in rare circumstances i have received unhandled error email,
+        # which somehow was triggered by 'fanstatic.needed' being missing
+        # from the environ.  not sure why that would happen, but it seems
+        # safe to ignore when it does - favicon is not *that* important.
+        if "fanstatic.needed" in request.environ:
+            return self.get_fanstatic_url(request, static.favicon)
+
+        return ""
 
     def get_header_logo_url(self, request):
         """
diff --git a/tests/test_handler.py b/tests/test_handler.py
index e7b0254..b038e47 100644
--- a/tests/test_handler.py
+++ b/tests/test_handler.py
@@ -34,9 +34,17 @@ class TestWebHandler(WebTestCase):
         self.assertEqual(url, "/fanstatic/wuttaweb_img/logo.png")
 
         # what about a subpath
-        self.request.script_name = "/testing"
-        url = handler.get_fanstatic_url(self.request, static.logo)
-        self.assertEqual(url, "/testing/fanstatic/wuttaweb_img/logo.png")
+        with patch.object(self.request, "script_name", new="/testing"):
+            url = handler.get_fanstatic_url(self.request, static.logo)
+            self.assertEqual(url, "/testing/fanstatic/wuttaweb_img/logo.png")
+
+        # error if environ missing config/data
+        environ = dict(self.request.environ)
+        del environ["fanstatic.needed"]
+        with patch.object(self.request, "environ", new=environ):
+            self.assertRaises(
+                KeyError, handler.get_fanstatic_url, self.request, static.logo
+            )
 
     def test_get_favicon_url(self):
         handler = self.make_handler()
@@ -45,6 +53,12 @@ class TestWebHandler(WebTestCase):
         url = handler.get_favicon_url(self.request)
         self.assertEqual(url, "/fanstatic/wuttaweb_img/favicon.ico")
 
+        # returns empty if environ missing config/data
+        environ = dict(self.request.environ)
+        del environ["fanstatic.needed"]
+        with patch.object(self.request, "environ", new=environ):
+            self.assertEqual(handler.get_favicon_url(self.request), "")
+
         # config override
         self.config.setdefault("wuttaweb.favicon_url", "/testing/other.ico")
         url = handler.get_favicon_url(self.request)