diff --git a/winpr/libwinpr/comm/comm_serial_sys.c b/winpr/libwinpr/comm/comm_serial_sys.c
index 975c514a4..950f36f6b 100644
--- a/winpr/libwinpr/comm/comm_serial_sys.c
+++ b/winpr/libwinpr/comm/comm_serial_sys.c
@@ -37,6 +37,12 @@
 #include <winpr/crt.h>
 #include <winpr/wlog.h>
 
+/* Undocumented flag, not supported everywhere.
+ * Provide a sensible fallback to avoid compilation problems. */
+#ifndef CMSPAR
+#define CMSPAR 010000000000
+#endif
+
 /* hard-coded in N_TTY */
 #define TTY_THRESHOLD_THROTTLE		128 /* now based on remaining room */
 #define TTY_THRESHOLD_UNTHROTTLE 	128