Program misbehaving on changing #define string

In my program i have used #define SWVER "V2.5.18" and the program is working fine. When i change this to #define SWVER "V2.5.18F" the program starts misbehaving. Next i created an error by forcibly creating an error by change #define SWVER V2.5.18F and compiled the code. Then i correct back to #define SWVER "V2.5.18F"  and recompiled. Now the program is working fine