diff --git a/src/bin/sage-runtests b/src/bin/sage-runtests
index 4d606c5a3a8..01b7ca44868 100755
--- a/src/bin/sage-runtests
+++ b/src/bin/sage-runtests
@@ -175,13 +175,18 @@ if __name__ == "__main__":
         pytest_options = []
         if args.verbose:
             pytest_options.append("-v")
-        # #31924: Do not run pytest on individual Python files unless
-        # they match the pytest file pattern.  However, pass names
-        # of directories. We use 'not os.path.isfile(f)' for this so that
-        # we do not silently hide typos.
-        filenames = [f for f in args.filenames
-                     if f.endswith("_test.py") or not os.path.isfile(f)]
-        if filenames or not args.filenames:
+
+        # #35999: no filename in arguments defaults to "src"
+        if not args.filenames:
+            filenames = [SAGE_SRC]
+        else:
+            # #31924: Do not run pytest on individual Python files unless
+            # they match the pytest file pattern.  However, pass names
+            # of directories. We use 'not os.path.isfile(f)' for this so that
+            # we do not silently hide typos.
+            filenames = [f for f in args.filenames
+                         if f.endswith("_test.py") or not os.path.isfile(f)]
+        if filenames:
             print(f"Running pytest on {filenames} with options {pytest_options}")
             exit_code_pytest = pytest.main(filenames + pytest_options)
             if exit_code_pytest == 5: