ERR_INVALID_ARG_TYPE(name, 'string', value)

[P-Vamshi-Seshasayan]

I want to display the link of teh generated slides on a php page using the following code. slides.md is already there. It's working fine in the terminal but not in php.
$output=array();
exec('"/home/user/.npm-global/bin/md2gslides" slides.md --copy=1z3XLhVWc03U3deHy --title "Test Slide" --no-browser 2>&1', $output);
print_r($output);

Any help is appreciated.

Error:
Array
(
[0] => internal/validators.js:125
[1] => throw new ERR_INVALID_ARG_TYPE(name, 'string', value);
[2] => ^
[3] =>
[4] => TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received type undefined
[5] => at validateString (internal/validators.js:125:11)
[6] => at Object.join (path.js:1147:7)
[7] => at Object. (/home/user/.npm-global/lib/node_modules/md2gslides/bin/md2gslides.js:33:38)
[8] => at Module._compile (internal/modules/cjs/loader.js:778:30)
[9] => at Object.Module._extensions..js (internal/modules/cjs/loader.js:789:10)
[10] => at Module.load (internal/modules/cjs/loader.js:653:32)
[11] => at tryModuleLoad (internal/modules/cjs/loader.js:593:12)
[12] => at Function.Module._load (internal/modules/cjs/loader.js:585:3)
[13] => at Function.Module.runMain (internal/modules/cjs/loader.js:831:12)
[14] => at startup (internal/bootstrap/node.js:283:19)
[15] => at bootstrapNodeJSCore (internal/bootstrap/node.js:623:3)
)