diff --git a/src/protocol/input_source.rs b/src/protocol/input_source.rs
index fccb0372e7697ff8478444038d3fad561cb7e0f1..f2cf10693ae87bc134610829d91344bcc1f221f1 100644
--- a/src/protocol/input_source.rs
+++ b/src/protocol/input_source.rs
@@ -21,9 +21,10 @@ pub struct InputSpan {
 }
 
 impl InputSpan {
-    // This will only be used for builtin functions
+    // This must only be used if you're sure that the span will not be involved
+    // in creating an error message.
     #[inline]
-    pub fn new() -> InputSpan {
+    pub const fn new() -> InputSpan {
         InputSpan{ begin: InputPosition{ line: 0, offset: 0 }, end: InputPosition{ line: 0, offset: 0 }}
     }