String Map command.. is there any bug..?

String map is widely used command. It helps user to manipulates many operations.
The syntax is as follows- string option arg ?arg ...?
For more details u can refer following link.
http://www.tcl.tk/man/tcl8.4/TclCmd/string.htm
I was using this string map to replace a string say 251 in following code from set of strings (contents of a).
My code was as follows-
set a "251 587 569 485"
set b "251"
set c [string map {$b " "} $a]
after executing the above set of lines i was expecting to get the following result-
i.e. content of variable c must change to " 587 569 485".
But it did not work. because the in string map variable reference is not working.
If i change the code as follows , then it will work-
set a "251 587 569 485"
set c [string map { 251 " "} $a]
Now the content of variable c will be " 587 569 485".
I don't know weather it is a bug or not.. :(

I approached other method to over come this problem.
I wrote following code
set a "251 587 569 485"
set b "251"
regexp -indices " $b " $a var;
c [string replace $a [lindex $var 0] [lindex $var 1] " "]
The above set of code replaces the variable b from variable a with null charecter. :)