diff --git a/config/src/lib.rs b/config/src/lib.rs
index a5afd4c..894ae7b 100644
--- a/config/src/lib.rs
+++ b/config/src/lib.rs
@@ -373,3 +373,35 @@ impl<'de> de::Deserialize<'de> for PathStr<'static> {
         deserializer.deserialize_str(PathStrVisitor)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_path_str_no_ref() {
+        let path_str: PathStr = "foo.bar.baz".parse().unwrap();
+        assert_eq!(path_str.module().to_string(), "foo.bar");
+        assert_eq!(path_str.name(), "baz");
+        assert_eq!(path_str.to_string(), "foo.bar.baz");
+        assert!(!path_str.has_ref());
+    }
+
+    #[test]
+    fn test_path_str_with_ref() {
+        let path_str: PathStr = "foo.$bar.baz".parse().unwrap();
+        assert_eq!(path_str.module().to_string(), "foo.$bar");
+        assert_eq!(path_str.name(), "baz");
+        assert_eq!(path_str.to_string(), "foo.$bar.baz");
+        assert!(path_str.has_ref());
+
+        let refs: RefMap = vec![("bar".to_string(), "qux.quux".parse().unwrap())]
+            .into_iter()
+            .collect();
+        let replaced = path_str.replace_refs(&refs).unwrap();
+        assert_eq!(replaced.module().to_string(), "foo.qux.quux");
+        assert_eq!(replaced.name(), "baz");
+        assert_eq!(replaced.to_string(), "foo.qux.quux.baz");
+        assert!(!replaced.has_ref());
+    }
+}
diff --git a/runner/src/pipeline.rs b/runner/src/pipeline.rs
index 1a551f6..202eb2d 100644
--- a/runner/src/pipeline.rs
+++ b/runner/src/pipeline.rs
@@ -403,16 +403,18 @@ impl Pipeline {
         env: &PyEnv,
         def: Option<&FunctionDef>,
     ) -> PyResult<LayerFunctionDef> {
-        Ok(match def {
-            Some(FunctionDef { path }) => {
-                if path.has_ref() {
-                    LayerFunctionDef::UseDefault
-                } else {
-                    LayerFunctionDef::Some(LayerFunction::new(py, env.import_path(py, path)?)?)
-                }
+        if let Some(FunctionDef { path }) = def {
+            if path.has_ref() {
+                Ok(LayerFunctionDef::UseDefault)
+            } else {
+                Ok(LayerFunctionDef::Some(LayerFunction::new(
+                    py,
+                    env.import_path(py, path)?,
+                )?))
             }
-            None => LayerFunctionDef::None,
-        })
+        } else {
+            Ok(LayerFunctionDef::None)
+        }
     }
 
     pub fn generator(&self) -> PyResult<impl Stream<Item = PyResult<PyObject>>> {