--- Include/pyport.h
+++ Include/pyport.h
@@ -219,7 +219,7 @@
 /* Smallest negative value of type Py_ssize_t. */
 #define PY_SSIZE_T_MIN (-PY_SSIZE_T_MAX-1)
 
-#if SIZEOF_PID_T > SIZEOF_LONG
+#if defined SIZEOF_PID_T && SIZEOF_PID_T > SIZEOF_LONG
 #   error "Python doesn't support sizeof(pid_t) > sizeof(long)"
 #endif