I'm running cl-json 0.3.1 and I seem to encounter the following issue on OpenMCL:

(encode-json-to-string '(("x" . "blah"))) => "[["x","l","a","h"]]"

On SBCL this works as expected:

(encode-json-to-string '(("x" . "blah"))) => "{"x":"blah"}"

I could use 'encode-json-alist-to-string', but I'll run into the same problem with inner alist elements of the list (I have an alist of alists).