cannot copy string correctly

I use following code:

StrCpy $INSTDIR $TARGETDIR
MessageBox MB_ICONQUESTION|MB_YESNO|MB_DEFBUTTON2 $TARGETDIR MessageBox MB_ICONQUESTION|MB_YESNO|MB_DEFBUTTON2 $INSTDIR 

Those two message boxes are for checking purpose only. Why is it so when e.g. in $TARGETDIR i have c:\BLEBLEBLE\zz
after copying in $INSTDIR i obtain c\BLEBLEBLE\zz - in other words why the colon character is not copied?

You probably have a space before C:\ in $TARGETDIR. $INSTDIR is always normalized so it'd contain a valid path.