diff --git a/clap_derive/examples/keyvalue.rs b/clap_derive/examples/keyvalue.rs
index 5bf952fb..c4724ca6 100644
--- a/clap_derive/examples/keyvalue.rs
+++ b/clap_derive/examples/keyvalue.rs
@@ -4,12 +4,12 @@ use clap::Clap;
 use std::error::Error;
 
 /// Parse a single key-value pair
-fn parse_key_val<T, U>(s: &str) -> Result<(T, U), Box<dyn Error>>
+fn parse_key_val<T, U>(s: &str) -> Result<(T, U), Box<dyn Error + Send + Sync + 'static>>
 where
     T: std::str::FromStr,
-    T::Err: Error + 'static,
+    T::Err: Error + Send + Sync + 'static,
     U: std::str::FromStr,
-    U::Err: Error + 'static,
+    U::Err: Error + Send + Sync + 'static,
 {
     let pos = s
         .find('=')
diff --git a/clap_derive/src/derives/into_app.rs b/clap_derive/src/derives/into_app.rs
index 4570f749..01baca22 100644
--- a/clap_derive/src/derives/into_app.rs
+++ b/clap_derive/src/derives/into_app.rs
@@ -227,7 +227,6 @@ pub fn gen_app_augmentation(
                         .validator(|s| {
                             #func(s)
                             .map(|_: #convert_type| ())
-                            .map_err(|e| e.to_string())
                         })
                     },
                     ParserKind::TryFromOsStr => quote_spanned! { func.span()=>
diff --git a/clap_derive/tests/custom-string-parsers.rs b/clap_derive/tests/custom-string-parsers.rs
index d127cac9..8c6f8ad0 100644
--- a/clap_derive/tests/custom-string-parsers.rs
+++ b/clap_derive/tests/custom-string-parsers.rs
@@ -90,7 +90,15 @@ fn test_parse_hex() {
 fn custom_parser_1(_: &str) -> &'static str {
     "A"
 }
-fn custom_parser_2(_: &str) -> Result<&'static str, u32> {
+#[derive(Debug)]
+struct ErrCode(u32);
+impl std::error::Error for ErrCode {}
+impl std::fmt::Display for ErrCode {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        std::fmt::Display::fmt(&self.0, f)
+    }
+}
+fn custom_parser_2(_: &str) -> Result<&'static str, ErrCode> {
     Ok("B")
 }
 fn custom_parser_3(_: &OsStr) -> &'static str {
diff --git a/src/build/arg/mod.rs b/src/build/arg/mod.rs
index a1cfd98e..1c555444 100644
--- a/src/build/arg/mod.rs
+++ b/src/build/arg/mod.rs
@@ -11,6 +11,7 @@ use std::{
     borrow::Cow,
     cmp::{Ord, Ordering},
     env,
+    error::Error,
     ffi::{OsStr, OsString},
     fmt::{self, Display, Formatter},
     str,
@@ -38,8 +39,8 @@ use crate::{
 #[cfg(feature = "yaml")]
 use yaml_rust::Yaml;
 
-type Validator<'a> = dyn FnMut(&str) -> Result<(), String> + Send + 'a;
-type ValidatorOs<'a> = dyn FnMut(&OsStr) -> Result<(), String> + Send + 'a;
+type Validator<'a> = dyn FnMut(&str) -> Result<(), Box<dyn Error + Send + Sync>> + Send + 'a;
+type ValidatorOs<'a> = dyn FnMut(&OsStr) -> Result<(), Box<dyn Error + Send + Sync>> + Send + 'a;
 
 /// The abstract representation of a command line argument. Used to set all the options and
 /// relationships that define a valid argument for the program.
@@ -1879,10 +1880,10 @@ impl<'help> Arg<'help> {
     pub fn validator<F, O, E>(mut self, mut f: F) -> Self
     where
         F: FnMut(&str) -> Result<O, E> + Send + 'help,
-        E: ToString,
+        E: Into<Box<dyn Error + Send + Sync + 'static>>,
     {
         self.validator = Some(Arc::new(Mutex::new(move |s: &str| {
-            f(s).map(|_| ()).map_err(|e| e.to_string())
+            f(s).map(|_| ()).map_err(|e| e.into())
         })));
         self
     }
@@ -1917,11 +1918,14 @@ impl<'help> Arg<'help> {
     /// [`Result`]: https://doc.rust-lang.org/std/result/enum.Result.html
     /// [`Err(String)`]: https://doc.rust-lang.org/std/result/enum.Result.html#variant.Err
     /// [`Rc`]: https://doc.rust-lang.org/std/rc/struct.Rc.html
-    pub fn validator_os<F, O>(mut self, mut f: F) -> Self
+    pub fn validator_os<F, O, E>(mut self, mut f: F) -> Self
     where
-        F: FnMut(&OsStr) -> Result<O, String> + Send + 'help,
+        F: FnMut(&OsStr) -> Result<O, E> + Send + 'help,
+        E: Into<Box<dyn Error + Send + Sync + 'static>>,
     {
-        self.validator_os = Some(Arc::new(Mutex::new(move |s: &OsStr| f(s).map(|_| ()))));
+        self.validator_os = Some(Arc::new(Mutex::new(move |s: &OsStr| {
+            f(s).map(|_| ()).map_err(|e| e.into())
+        })));
         self
     }
 
diff --git a/src/parse/errors.rs b/src/parse/errors.rs
index e44b6807..dfd01a7c 100644
--- a/src/parse/errors.rs
+++ b/src/parse/errors.rs
@@ -385,6 +385,7 @@ pub struct Error {
     /// Additional information depending on the error kind, like values and argument names.
     /// Useful when you want to render an error of your own.
     pub info: Vec<String>,
+    pub(crate) source: Option<Box<dyn error::Error + Send + Sync>>,
 }
 
 impl Display for Error {
@@ -478,6 +479,7 @@ impl Error {
             message: c,
             kind: ErrorKind::ArgumentConflict,
             info,
+            source: None,
         }
     }
 
@@ -495,6 +497,7 @@ impl Error {
             message: c,
             kind: ErrorKind::EmptyValue,
             info: vec![arg],
+            source: None,
         }
     }
 
@@ -557,6 +560,7 @@ impl Error {
             message: c,
             kind: ErrorKind::InvalidValue,
             info: vec![],
+            source: None,
         }
     }
 
@@ -587,6 +591,7 @@ impl Error {
             message: c,
             kind: ErrorKind::InvalidSubcommand,
             info: vec![subcmd],
+            source: None,
         }
     }
 
@@ -608,6 +613,7 @@ impl Error {
             message: c,
             kind: ErrorKind::UnrecognizedSubcommand,
             info: vec![subcmd],
+            source: None,
         }
     }
 
@@ -637,6 +643,7 @@ impl Error {
             message: c,
             kind: ErrorKind::MissingRequiredArgument,
             info,
+            source: None,
         }
     }
 
@@ -653,6 +660,7 @@ impl Error {
             message: c,
             kind: ErrorKind::MissingSubcommand,
             info: vec![],
+            source: None,
         }
     }
 
@@ -670,6 +678,7 @@ impl Error {
             message: c,
             kind: ErrorKind::InvalidUtf8,
             info: vec![],
+            source: None,
         }
     }
 
@@ -693,6 +702,7 @@ impl Error {
             message: c,
             kind: ErrorKind::TooManyValues,
             info: vec![arg.to_string(), val],
+            source: None,
         }
     }
 
@@ -720,13 +730,14 @@ impl Error {
             message: c,
             kind: ErrorKind::TooFewValues,
             info: vec![arg.to_string(), curr_vals.to_string(), min_vals.to_string()],
+            source: None,
         }
     }
 
     pub(crate) fn value_validation(
         arg: String,
         val: String,
-        err: String,
+        err: Box<dyn error::Error + Send + Sync>,
         color: ColorChoice,
     ) -> Self {
         let mut c = Colorizer::new(true, color);
@@ -743,7 +754,8 @@ impl Error {
         Error {
             message: c,
             kind: ErrorKind::ValueValidation,
-            info: vec![arg, val, err],
+            info: vec![arg, val, err.to_string()],
+            source: Some(err),
         }
     }
 
@@ -771,6 +783,7 @@ impl Error {
             message: c,
             kind: ErrorKind::WrongNumberOfValues,
             info: vec![arg.to_string(), curr_vals.to_string(), num_vals.to_string()],
+            source: None,
         }
     }
 
@@ -788,6 +801,7 @@ impl Error {
             message: c,
             kind: ErrorKind::UnexpectedMultipleUsage,
             info: vec![arg],
+            source: None,
         }
     }
 
@@ -836,6 +850,7 @@ impl Error {
             message: c,
             kind: ErrorKind::UnknownArgument,
             info: vec![arg],
+            source: None,
         }
     }
 
@@ -857,6 +872,7 @@ impl Error {
             message: c,
             kind: ErrorKind::UnknownArgument,
             info: vec![arg],
+            source: None,
         }
     }
 
@@ -872,6 +888,7 @@ impl Error {
             message: c,
             kind: ErrorKind::ArgumentNotFound,
             info: vec![arg],
+            source: None,
         }
     }
 
@@ -888,6 +905,7 @@ impl Error {
             message: c,
             kind,
             info: vec![],
+            source: None,
         }
     }
 }
@@ -904,4 +922,12 @@ impl From<fmt::Error> for Error {
     }
 }
 
-impl error::Error for Error {}
+impl error::Error for Error {
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        if let Some(source) = self.source.as_deref() {
+            Some(source)
+        } else {
+            None
+        }
+    }
+}
diff --git a/src/parse/matches/arg_matches.rs b/src/parse/matches/arg_matches.rs
index 3fc3362b..c7d06e69 100644
--- a/src/parse/matches/arg_matches.rs
+++ b/src/parse/matches/arg_matches.rs
@@ -351,7 +351,12 @@ impl ArgMatches {
                     v, name, e
                 );
 
-                Error::value_validation(name.to_string(), v.to_string(), message, ColorChoice::Auto)
+                Error::value_validation(
+                    name.to_string(),
+                    v.to_string(),
+                    message.into(),
+                    ColorChoice::Auto,
+                )
             })
         } else {
             Err(Error::argument_not_found_auto(name.to_string()))
@@ -439,7 +444,7 @@ impl ArgMatches {
                     Error::value_validation(
                         name.to_string(),
                         v.to_string(),
-                        message,
+                        message.into(),
                         ColorChoice::Auto,
                     )
                 })
diff --git a/src/parse/parser.rs b/src/parse/parser.rs
index 1df579fd..47bbc2e0 100644
--- a/src/parse/parser.rs
+++ b/src/parse/parser.rs
@@ -752,6 +752,7 @@ impl<'help, 'app> Parser<'help, 'app> {
                     message,
                     kind: ErrorKind::DisplayHelpOnMissingArgumentOrSubcommand,
                     info: vec![],
+                    source: None,
                 });
             }
         }
@@ -1747,6 +1748,7 @@ impl<'help, 'app> Parser<'help, 'app> {
                 message: c,
                 kind: ErrorKind::DisplayHelp,
                 info: vec![],
+                source: None,
             },
         }
     }
@@ -1761,6 +1763,7 @@ impl<'help, 'app> Parser<'help, 'app> {
             message: c,
             kind: ErrorKind::DisplayVersion,
             info: vec![],
+            source: None,
         }
     }
 }
diff --git a/src/parse/validator.rs b/src/parse/validator.rs
index 8f63d525..2ba7c798 100644
--- a/src/parse/validator.rs
+++ b/src/parse/validator.rs
@@ -62,6 +62,7 @@ impl<'help, 'app, 'parser> Validator<'help, 'app, 'parser> {
                 message,
                 kind: ErrorKind::DisplayHelpOnMissingArgumentOrSubcommand,
                 info: vec![],
+                source: None,
             });
         }
         self.validate_conflicts(matcher)?;