diff --git a/tests/conftest.py b/tests/conftest.py
index 1a63a5d2..9f796d3a 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -205,6 +205,8 @@ def run_ocrmypdf_api(input_file, output_file, *args, env=None):
     if env:
         options.tesseract_env = env
         options.tesseract_env['_OCRMYPDF_TEST_INFILE'] = os.fspath(input_file)
+    if options.tesseract_env:
+        assert all(isinstance(v, (str, bytes)) for v in options.tesseract_env.values())
 
     return api.run_pipeline(options, api=False)
 
diff --git a/tests/test_main.py b/tests/test_main.py
index 77ea154e..17946965 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -303,7 +303,7 @@ def test_maximum_options(
 
 def test_tesseract_missing_tessdata(resources, no_outpdf, tmpdir):
     env = os.environ.copy()
-    env['TESSDATA_PREFIX'] = tmpdir
+    env['TESSDATA_PREFIX'] = os.fspath(tmpdir)
 
     returncode = run_ocrmypdf_api(
         resources / 'graph.pdf', no_outpdf, '-v', '1', '--skip-text', env=env