diff --git a/CHANGES b/CHANGES
index a11e4800f..47fa81e6a 100644
--- a/CHANGES
+++ b/CHANGES
@@ -41,6 +41,7 @@ Bugs fixed
 * #5502: linkcheck: Consider HTTP 503 response as not an error
 * #6439: Make generated download links reproducible
 * #6486: UnboundLocalError is raised if broken extension installed
+* #6498: autosummary: crashed with wrong autosummary_generate setting
 
 Testing
 --------
diff --git a/sphinx/ext/autosummary/__init__.py b/sphinx/ext/autosummary/__init__.py
index 6eb0fea9b..19c400609 100644
--- a/sphinx/ext/autosummary/__init__.py
+++ b/sphinx/ext/autosummary/__init__.py
@@ -58,6 +58,7 @@ import posixpath
 import re
 import sys
 import warnings
+from os import path
 from types import ModuleType
 from typing import List, cast
 
@@ -731,26 +732,31 @@ def process_generate_options(app):
     # type: (Sphinx) -> None
     genfiles = app.config.autosummary_generate
 
-    if genfiles and not hasattr(genfiles, '__len__'):
+    if genfiles is True:
         env = app.builder.env
         genfiles = [env.doc2path(x, base=None) for x in env.found_docs
                     if os.path.isfile(env.doc2path(x))]
+    else:
+        ext = list(app.config.source_suffix)
+        genfiles = [genfile + (not genfile.endswith(tuple(ext)) and ext[0] or '')
+                    for genfile in genfiles]
+
+        for entry in genfiles[:]:
+            if not path.isfile(path.join(app.srcdir, entry)):
+                logger.warning(__('autosummary_generate: file not found: %s'), entry)
+                genfiles.remove(entry)
 
     if not genfiles:
         return
 
-    from sphinx.ext.autosummary.generate import generate_autosummary_docs
-
-    ext = list(app.config.source_suffix)
-    genfiles = [genfile + (not genfile.endswith(tuple(ext)) and ext[0] or '')
-                for genfile in genfiles]
-
     suffix = get_rst_suffix(app)
     if suffix is None:
         logger.warning(__('autosummary generats .rst files internally. '
                           'But your source_suffix does not contain .rst. Skipped.'))
         return
 
+    from sphinx.ext.autosummary.generate import generate_autosummary_docs
+
     imported_members = app.config.autosummary_imported_members
     with mock(app.config.autosummary_mock_imports):
         generate_autosummary_docs(genfiles, builder=app.builder,
diff --git a/tests/test_ext_autosummary.py b/tests/test_ext_autosummary.py
index ae97d3b57..ae2b2d585 100644
--- a/tests/test_ext_autosummary.py
+++ b/tests/test_ext_autosummary.py
@@ -302,3 +302,9 @@ def test_generate_autosummary_docs_property(app):
                        ".. currentmodule:: target.methods\n"
                        "\n"
                        ".. autoproperty:: Base.prop")
+
+
+@pytest.mark.sphinx('dummy', testroot='ext-autosummary',
+                    confoverrides={'autosummary_generate': ['unknown']})
+def test_invalid_autosummary_generate(app, status, warning):
+    assert 'WARNING: autosummary_generate: file not found: unknown.rst' in warning.getvalue()