Only check the exit status if the process has actually exited, otherwise

it will return a nil exit status.

lib/cocoapods/generator/documentation.rb

@@ -102,7 +102,7 @@ module Pod
         end
 
         # appledoc exits with 1 if a warning was logged
-        if (process.exitstatus >= 2) && !config.silent?
+        if process.exited? && (process.exitstatus >= 2) && !config.silent?
           puts "[!] Appledoc encountered an error. Run 'pod install --verbose' for details."
         end
       end