explicitly throw an error when the last argument is not a function

index.js

@@ -54,6 +54,15 @@ module.exports = function execPodCallback(subcommand, args, options, cb) {
     cmd = 'pod';
   }
 
+  if (typeof cb !== 'function') {
+    throw new TypeError(
+      String(cb) +
+      ' is not a function. Expected a callback function called after `' +
+      [cmd].concat(subcommand, args).join(' ') +
+      '` command runs.'
+    );
+  }
+
   return execFileSubcommand(cmd, subcommand, args, options, function callback(err, stdout, stderr) {
     if (err) {
       err.message = err.message.replace('Command failed: ' + err.cmd + '\n', '$&' + stdout + stderr);

test.js

@@ -4,7 +4,7 @@ const execPodCallback = require('.');
 const test = require('tape');
 
 test('execPodCallback()', t => {
-  t.plan(17);
+  t.plan(19);
 
   t.equal(execPodCallback.name, 'execPodCallback', 'should have a function name.');
 
@@ -96,4 +96,16 @@ test('execPodCallback()', t => {
     /TypeError.*1 is not Boolean\. `bundleExec` option must be Boolean \(`false` by default\)\./,
     'should throw a type error when `bundleExec` option is not Boolean.'
   );
+
+  t.throws(
+    () => execPodCallback('install', ['AFNetworking', '--verbose'], false),
+    /TypeError.*callback function called after `pod install AFNetworking --verbose` command runs\./,
+    'should throw a type error when the last argument is falsy value except for `undefined`.'
+  );
+
+  t.throws(
+    () => execPodCallback('install', [], 1),
+    /TypeError.*1 is not a function\. Expected a callback function called after `pod install`/,
+    'should throw a type error when the last argument is truthy value but not a function.'
+  );
 });