diff --git a/src/assert.rs b/src/assert.rs
index 0a78f5c..e4a43f4 100644
--- a/src/assert.rs
+++ b/src/assert.rs
@@ -4,7 +4,7 @@ use std::path::PathBuf;
 use std::vec::Vec;
 
 use errors::*;
-use output::{OutputAssertion, StdErr, StdOut};
+use output::{OutputAssertion, OutputKind};
 
 /// Assertions for a specific command.
 #[derive(Debug)]
@@ -13,8 +13,7 @@ pub struct Assert {
     current_dir: Option<PathBuf>,
     expect_success: Option<bool>,
     expect_exit_code: Option<i32>,
-    expect_stdout: Vec<OutputAssertion<StdOut>>,
-    expect_stderr: Vec<OutputAssertion<StdErr>>,
+    expect_output: Vec<OutputAssertion>,
 }
 
 impl default::Default for Assert {
@@ -28,8 +27,7 @@ impl default::Default for Assert {
             current_dir: None,
             expect_success: Some(true),
             expect_exit_code: None,
-            expect_stdout: vec![],
-            expect_stderr: vec![],
+            expect_output: vec![],
         }
     }
 }
@@ -190,11 +188,11 @@ impl Assert {
     ///     .unwrap();
     /// ```
     pub fn prints<O: Into<String>>(mut self, output: O) -> Self {
-        self.expect_stdout.push(OutputAssertion {
+        self.expect_output.push(OutputAssertion {
             expect: output.into(),
             fuzzy: true,
             expected_result: true,
-            kind: StdOut,
+            kind: OutputKind::StdOut,
         });
         self
     }
@@ -211,11 +209,11 @@ impl Assert {
     ///     .unwrap();
     /// ```
     pub fn prints_exactly<O: Into<String>>(mut self, output: O) -> Self {
-        self.expect_stdout.push(OutputAssertion {
+        self.expect_output.push(OutputAssertion {
             expect: output.into(),
             fuzzy: false,
             expected_result: true,
-            kind: StdOut,
+            kind: OutputKind::StdOut,
         });
         self
     }
@@ -234,11 +232,11 @@ impl Assert {
     ///     .unwrap();
     /// ```
     pub fn prints_error<O: Into<String>>(mut self, output: O) -> Self {
-        self.expect_stderr.push(OutputAssertion {
+        self.expect_output.push(OutputAssertion {
             expect: output.into(),
             fuzzy: true,
             expected_result: true,
-            kind: StdErr,
+            kind: OutputKind::StdErr,
         });
         self
     }
@@ -257,11 +255,11 @@ impl Assert {
     ///     .unwrap();
     /// ```
     pub fn prints_error_exactly<O: Into<String>>(mut self, output: O) -> Self {
-        self.expect_stderr.push(OutputAssertion {
+        self.expect_output.push(OutputAssertion {
             expect: output.into(),
             fuzzy: false,
             expected_result: true,
-            kind: StdErr,
+            kind: OutputKind::StdErr,
         });
         self
     }
@@ -279,11 +277,11 @@ impl Assert {
     ///     .unwrap();
     /// ```
     pub fn doesnt_print<O: Into<String>>(mut self, output: O) -> Self {
-        self.expect_stdout.push(OutputAssertion {
+        self.expect_output.push(OutputAssertion {
             expect: output.into(),
             fuzzy: true,
             expected_result: false,
-            kind: StdOut,
+            kind: OutputKind::StdOut,
         });
         self
     }
@@ -301,11 +299,11 @@ impl Assert {
     ///     .unwrap();
     /// ```
     pub fn doesnt_print_exactly<O: Into<String>>(mut self, output: O) -> Self {
-        self.expect_stdout.push(OutputAssertion {
+        self.expect_output.push(OutputAssertion {
             expect: output.into(),
             fuzzy: false,
             expected_result: false,
-            kind: StdOut,
+            kind: OutputKind::StdOut,
         });
         self
     }
@@ -325,11 +323,11 @@ impl Assert {
     ///     .unwrap();
     /// ```
     pub fn doesnt_print_error<O: Into<String>>(mut self, output: O) -> Self {
-        self.expect_stderr.push(OutputAssertion {
+        self.expect_output.push(OutputAssertion {
             expect: output.into(),
             fuzzy: true,
             expected_result: false,
-            kind: StdErr,
+            kind: OutputKind::StdErr,
         });
         self
     }
@@ -349,11 +347,11 @@ impl Assert {
     ///     .unwrap();
     /// ```
     pub fn doesnt_print_error_exactly<O: Into<String>>(mut self, output: O) -> Self {
-        self.expect_stderr.push(OutputAssertion {
+        self.expect_output.push(OutputAssertion {
             expect: output.into(),
             fuzzy: false,
             expected_result: false,
-            kind: StdErr,
+            kind: OutputKind::StdErr,
         });
         self
     }
@@ -399,13 +397,9 @@ impl Assert {
             ));
         }
 
-        self.expect_stdout
+        self.expect_output
             .iter()
-            .map(|a| a.execute(&output).map_err(|e| ErrorKind::StdoutMismatch(self.cmd.clone(), e).into()))
-            .collect::<Result<Vec<()>>>()?;
-        self.expect_stderr
-            .iter()
-            .map(|a| a.execute(&output).map_err(|e| ErrorKind::StderrMismatch(self.cmd.clone(), e).into()))
+            .map(|a| a.execute(&output, &self.cmd))
             .collect::<Result<Vec<()>>>()?;
 
         Ok(())
diff --git a/src/output.rs b/src/output.rs
index b95fee2..3376226 100644
--- a/src/output.rs
+++ b/src/output.rs
@@ -1,4 +1,3 @@
-use std::fmt;
 use std::process::Output;
 
 use difference::Changeset;
@@ -8,14 +7,14 @@ pub use self::errors::{Error, ErrorKind};
 use diff;
 
 #[derive(Debug, Clone)]
-pub struct OutputAssertion<T> {
+pub struct OutputAssertion {
     pub expect: String,
     pub fuzzy: bool,
     pub expected_result: bool,
-    pub kind: T,
+    pub kind: OutputKind,
 }
 
-impl<T: OutputType> OutputAssertion<T> {
+impl OutputAssertion {
     fn matches_fuzzy(&self, got: &str) -> Result<()> {
         let result = got.contains(&self.expect);
         if result != self.expected_result {
@@ -45,51 +44,37 @@ impl<T: OutputType> OutputAssertion<T> {
         Ok(())
     }
 
-    pub fn execute(&self, output: &Output) -> Result<()> {
+    pub fn execute(&self, output: &Output, cmd: &[String]) -> super::errors::Result<()> {
         let observed = String::from_utf8_lossy(self.kind.select(output));
 
-        if self.fuzzy {
+        let result = if self.fuzzy {
             self.matches_fuzzy(&observed)
         } else {
             self.matches_exact(&observed)
-        }
+        };
+        result.map_err(|e| self.kind.map_err(e, cmd))
     }
 }
 
-
-pub trait OutputType: fmt::Display {
-    fn select<'a>(&self, o: &'a Output) -> &'a [u8];
-}
-
-
 #[derive(Debug, Clone, Copy)]
-pub struct StdOut;
-
-impl fmt::Display for StdOut {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "stdout")
-    }
-}
-
-impl OutputType for StdOut {
-    fn select<'a>(&self, o: &'a Output) -> &'a [u8] {
-        &o.stdout
-    }
+pub enum OutputKind {
+    StdOut,
+    StdErr,
 }
 
-
-#[derive(Debug, Clone, Copy)]
-pub struct StdErr;
-
-impl fmt::Display for StdErr {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        write!(f, "stderr")
+impl OutputKind {
+    pub fn select<'a>(self, o: &'a Output) -> &'a [u8] {
+        match self {
+            OutputKind::StdOut => &o.stdout,
+            OutputKind::StdErr => &o.stderr,
+        }
     }
-}
 
-impl OutputType for StdErr {
-    fn select<'a>(&self, o: &'a Output) -> &'a [u8] {
-        &o.stderr
+    pub fn map_err(self, e: Error, cmd: &[String]) -> super::errors::Error {
+        match self {
+            OutputKind::StdOut => super::errors::ErrorKind::StdoutMismatch(cmd.to_vec(), e).into(),
+            OutputKind::StdErr => super::errors::ErrorKind::StderrMismatch(cmd.to_vec(), e).into(),
+        }
     }
 }