diff --git a/bin/tests/system/isctest/template.py b/bin/tests/system/isctest/template.py
index 6c6b628b94..83e6ce8691 100644
--- a/bin/tests/system/isctest/template.py
+++ b/bin/tests/system/isctest/template.py
@@ -40,10 +40,16 @@ class TemplateEngine:
         self.directory = Path(directory)
         self.env_vars = dict(env_vars)
         self.j2env = jinja2.Environment(
-            loader=jinja2.FileSystemLoader(
+            loader=jinja2.ChoiceLoader(
                 [
-                    str(self.directory),
-                    str(ALL["srcdir"]),  # to allow _common/ includes
+                    jinja2.FileSystemLoader(self.directory),
+                    jinja2.PrefixLoader(
+                        {
+                            "_common": jinja2.FileSystemLoader(
+                                Path(ALL["srcdir"]) / "_common"
+                            ),
+                        }
+                    ),
                 ]
             ),
             undefined=jinja2.StrictUndefined,