diff --git a/src/unstable.rs b/src/unstable.rs
index 730c5e7..ca13edf 100644
--- a/src/unstable.rs
+++ b/src/unstable.rs
@@ -287,7 +287,7 @@
 
 impl SourceFile {
     fn nightly(sf: proc_macro::SourceFile) -> Self {
-        let filename = stable::file_name(sf.path().to_string());
+        let filename = stable::file_name(sf.path().display().to_string());
         SourceFile::Nightly(sf, filename)
     }