diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 3d3f6ac..0163b07 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6243,9 +6243,10 @@
 /*[clinic end generated code: output=3626c81f98985d49 input=13c956413110eeaa]*/
 {
     pid_t pid;
-
-    if (_PyInterpreterState_GET() != PyInterpreterState_Main()) {
-        PyErr_SetString(PyExc_RuntimeError, "fork not supported for subinterpreters");
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    if (interp->config._isolated_interpreter) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "fork not supported for isolated subinterpreters");
         return NULL;
     }
     if (PySys_Audit("os.fork", NULL) < 0) {