not able to convert proving key json to bin

I produced the proving key json using zkutil. 
But not able to convert proving key json to bin
```
/zku/a1/node_modules/websnark/tools/buildpkey.js:68
    writeBigInt(h, toMontgomeryQ(p[0]));
                                  ^

TypeError: Cannot read property '0' of undefined
    at writePoint (/zku/a1/node_modules/websnark/tools/buildpkey.js:68:35)
```