diff --git a/src/comp/syntax/codemap.rs b/src/comp/syntax/codemap.rs
index 8cdbcd32caef..f08bec73a3d6 100644
--- a/src/comp/syntax/codemap.rs
+++ b/src/comp/syntax/codemap.rs
@@ -35,15 +35,17 @@ type lookup_fn = fn(file_pos) -> uint;
 
 fn lookup_pos(map: codemap, pos: uint, lookup: lookup_fn) -> loc {
     let len = vec::len(map.files);
+    if len > 1u && map.files[len - 1u].name == "-" {
+        // the trailing "-" must be the core_macros inserted by expand_crate,
+        // exclude it from the targets to lookup
+        len = len - 1u;
+    }
     let a = 0u;
     let b = len;
     while b - a > 1u {
         let m = (a + b) / 2u;
         if lookup(map.files[m].start_pos) > pos { b = m; } else { a = m; }
     }
-    if (a >= len) {
-        ret { filename: "-", line: 0u, col: 0u };
-    }
     let f = map.files[a];
     a = 0u;
     b = vec::len(f.lines);