diff --git a/FreeModbus/modbus/functions/mbfunccoils_m.c b/FreeModbus/modbus/functions/mbfunccoils_m.c
index f6bb51c..93801de 100644
--- a/FreeModbus/modbus/functions/mbfunccoils_m.c
+++ b/FreeModbus/modbus/functions/mbfunccoils_m.c
@@ -196,7 +196,7 @@ eMBMasterReqWriteCoil( UCHAR ucSndAddr, USHORT usCoilAddr, USHORT usCoilData, LO
     UCHAR                 *ucMBFrame;
     eMBMasterReqErrCode    eErrStatus = MB_MRE_NO_ERR;
 
-    if ( usCoilAddr > MB_MASTER_TOTAL_SLAVE_NUM ) eErrStatus = MB_MRE_ILL_ARG;
+    if ( ucSndAddr > MB_MASTER_TOTAL_SLAVE_NUM ) eErrStatus = MB_MRE_ILL_ARG;
     else if ( ( usCoilData != 0xFF00 ) && ( usCoilData != 0x0000 ) ) eErrStatus = MB_MRE_ILL_ARG;
     else if ( xMBMasterRunResTake( lTimeOut ) == FALSE ) eErrStatus = MB_MRE_MASTER_BUSY;
     else