diff --git a/RELEASES.md b/RELEASES.md
index 17e19f7d..ef71aa12 100644
--- a/RELEASES.md
+++ b/RELEASES.md
@@ -5,6 +5,11 @@ Rhai Release Notes
 Version 0.19.4
 ==============
 
+Bug fixes
+---------
+
+* Fixes `Send + Sync` for `EvalAltResult` under the `sync` feature. Bug introduced with `0.19.3`.
+  
 New features
 ------------
 
diff --git a/src/result.rs b/src/result.rs
index e5faeeb8..97c14e2d 100644
--- a/src/result.rs
+++ b/src/result.rs
@@ -25,7 +25,11 @@ use crate::stdlib::{
 #[non_exhaustive]
 pub enum EvalAltResult {
     /// System error. Wrapped values are the error message and the internal error.
+    #[cfg(not(feature = "sync"))]
     ErrorSystem(String, Box<dyn Error>),
+    /// System error. Wrapped values are the error message and the internal error.
+    #[cfg(feature = "sync")]
+    ErrorSystem(String, Box<dyn Error + Send + Sync>),
 
     /// Syntax error.
     ErrorParsing(ParseErrorType, Position),