diff --git a/spwplugin/SpwTcpPacketServer/spwtcppacketserver.cpp b/spwplugin/SpwTcpPacketServer/spwtcppacketserver.cpp
--- a/spwplugin/SpwTcpPacketServer/spwtcppacketserver.cpp
+++ b/spwplugin/SpwTcpPacketServer/spwtcppacketserver.cpp
@@ -157,7 +157,9 @@ void SpwTcpPacketServer::sendSPWPacketUs
     onePacketReceived();
     if(data[0]==(char)0)  // Protocole = 0 => Host to SpaceWire packet transmission
     {
-        int size = (data[1]*256*256) + (data[2]*256) + data[3];
+        int size = ( (unsigned char) data[1] ) * 256 * 256
+                 + ( (unsigned char) data[2] ) * 256
+                 +   (unsigned char) data[3];
         char* SPWpacket = (char*)malloc(size);
         memcpy(SPWpacket,data.data()+4,size);   // 4 bytes will be added later to the packet
         emit sendSPWPacket(SPWpacket,size);