diff --git a/examples/nop-preprocessor.rs b/examples/nop-preprocessor.rs
index df8bd757..7c04417a 100644
--- a/examples/nop-preprocessor.rs
+++ b/examples/nop-preprocessor.rs
@@ -1,9 +1,10 @@
 extern crate mdbook;
+extern crate serde_json;
 #[macro_use]
 extern crate clap;
 
 use clap::{App, Arg, ArgMatches, SubCommand};
-use mdbook::preprocess::{Preprocessor, PreprocessorContext, CmdPreprocessor};
+use mdbook::preprocess::CmdPreprocessor;
 use std::process;
 use std::io;
 
@@ -21,6 +22,8 @@ fn handle_preprocessing(args: &ArgMatches) {
     let (ctx, book) = CmdPreprocessor::parse_input(io::stdin())
         .expect("Couldn't parse the input");
 
+    eprintln!("{:?}", ctx.config);
+
     // You can tell the preprocessor to blow up by setting a particular
     // config value
     if let Some(table) = ctx.config.get_preprocessor("nop-preprocessor") {
@@ -30,6 +33,8 @@ fn handle_preprocessing(args: &ArgMatches) {
             panic!("Boom!!!1!");
         }
     }
+
+    serde_json::to_writer(io::stdout(), &book).unwrap();
 }
 
 fn handle_supports(sub_args: &ArgMatches) {
diff --git a/src/preprocess/cmd.rs b/src/preprocess/cmd.rs
index 2fc14ffc..81f8286e 100644
--- a/src/preprocess/cmd.rs
+++ b/src/preprocess/cmd.rs
@@ -105,7 +105,7 @@ impl Preprocessor for CmdPreprocessor {
             Ok(c) => c,
             Err(e) => {
                 warn!("Unable to create the command for the \"{}\" preprocessor, {}", self.name(), e);
-                return true;
+                return false;
             }
         };
 
diff --git a/tests/custom_preprocessors.rs b/tests/custom_preprocessors.rs
index d0182e7e..cfc0d1b3 100644
--- a/tests/custom_preprocessors.rs
+++ b/tests/custom_preprocessors.rs
@@ -32,7 +32,7 @@ fn ask_the_preprocessor_to_blow_up() {
     let mut md = MDBook::load(&dummy_book).unwrap();
     md.with_preprecessor(example());
 
-    md.config.set("preprocess.nop-preprocessor.blow-up", true).unwrap();
+    md.config.set("preprocessor.nop-preprocessor.blow-up", true).unwrap();
 
     let got = md.build();