strformat: using a ':' in a string literal possible?

[MaineTim]

Another newbie question: is it possible to embed a ':' in a string literal in a 
fmt specification, as in: 
    
    
     echo fmt("{\"This is a string:\":<30}) 
    
    Run

yields
    
    
     /usercode/in.nim(3, 10) Error: could not parse `"This is a string`.
     /nim/lib/core/macros.nim(540, 1) Error: closing " expected 
    
    Run

Attempting to "escape" the colon: 
    
    
     echo fmt("{\"This is a string\:\":<30}) 
    
    Run

yields 
    
    
     /usercode/in.nim(3, 31) Error: invalid character constant 
    
    Run

How can I sneak a colon in there short of assigning the string to a variable?

Thanks!