diff --git a/libraries/USBPD_SINK/examples/usbpd_sink_request_voltage/usbpd_sink_request_voltage.ino b/libraries/USBPD_SINK/examples/usbpd_sink_request_voltage/usbpd_sink_request_voltage.ino
index 6193386d..d9d82ab2 100644
--- a/libraries/USBPD_SINK/examples/usbpd_sink_request_voltage/usbpd_sink_request_voltage.ino
+++ b/libraries/USBPD_SINK/examples/usbpd_sink_request_voltage/usbpd_sink_request_voltage.ino
@@ -20,7 +20,10 @@ void loop() {
 
     if(usbpd_sink_get_ready())
     {
-        usbpd_sink_set_request_fixed_voltage(setVoltage);
+        if(usbpd_sink_set_request_fixed_voltage(setVoltage) == false)
+        {
+            Serial.printf("unsupported voltage\r\n");
+        }
     }
 
 // button, myIndex++
diff --git a/libraries/USBPD_SINK/src/usbpd_sink.c b/libraries/USBPD_SINK/src/usbpd_sink.c
index 0292e38b..82f166b1 100644
--- a/libraries/USBPD_SINK/src/usbpd_sink.c
+++ b/libraries/USBPD_SINK/src/usbpd_sink.c
@@ -38,7 +38,7 @@ void usbpd_sink_clear_ready(void)
     pdControl_g.cc_USBPD_READY = 0;
 }
 
-void usbpd_sink_set_request_fixed_voltage(Request_voltage_t requestVoltage)
+bool usbpd_sink_set_request_fixed_voltage(Request_voltage_t requestVoltage)
 {
     uint16_t targetVoltage;
     switch (requestVoltage)
@@ -73,11 +73,12 @@ void usbpd_sink_set_request_fixed_voltage(Request_voltage_t requestVoltage)
         if(pdControl_g.cc_FixedSourceCap[i].Voltage == targetVoltage)
         { 
             pdControl_g.cc_SetPDONum = i+1;
-            return;
+            return true;
         }
     }
-    pdControl_g.cc_SetPDONum = (pdControl_g.cc_SourcePDONum - pdControl_g.cc_SourcePPSNum);
     
+    // unsupported voltage
+    return false;
 }
 
 void timer3_init(uint16_t arr, uint16_t psc)
diff --git a/libraries/USBPD_SINK/src/usbpd_sink.h b/libraries/USBPD_SINK/src/usbpd_sink.h
index 1672d048..12dcc278 100644
--- a/libraries/USBPD_SINK/src/usbpd_sink.h
+++ b/libraries/USBPD_SINK/src/usbpd_sink.h
@@ -5,6 +5,7 @@
 extern "C" {
 #endif /* end of __cplusplus */
 
+#include <stdbool.h>
 #include "usbpd_def.h"
 
 // Register Bit Definition
@@ -201,7 +202,7 @@ void usbpd_sink_process(void);
 uint8_t usbpd_sink_get_ready(void);
 void usbpd_sink_clear_ready(void);
 
-void usbpd_sink_set_request_fixed_voltage(Request_voltage_t requestVoltage);
+bool usbpd_sink_set_request_fixed_voltage(Request_voltage_t requestVoltage);
 
 
 #ifdef __cplusplus