diff --git a/src/protocol.rs b/src/protocol.rs
index 3722367..94c9708 100644
--- a/src/protocol.rs
+++ b/src/protocol.rs
@@ -172,7 +172,7 @@ If you can read it, you can source it,
 and it's not necessary to read the whole thing--meaning you should be able to
 support SourceCodes which are gigabytes or larger in size.
 */
-pub trait SourceCode {
+pub trait SourceCode: Send + Sync {
     /// Read the bytes for a specific span from this SourceCode, keeping a
     /// certain number of lines before and after the span as context.
     fn read_span<'a>(