diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 59f0d2224..2bdb3bd96 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -699,11 +699,8 @@ SourcePath resolveExprPath(const SourcePath & path)
     #endif
 
     // FIXME
-    auto path2 = path.path + "/default.nix";
-    if (path.pathExists())
-        return {path.accessor, path2};
-
-    return path;
+    auto path2 = path.append("/default.nix");
+    return path2.pathExists() ? path2 : path;
 }
 
 
diff --git a/src/libfetchers/input-accessor.cc b/src/libfetchers/input-accessor.cc
index 10a275857..c4743c201 100644
--- a/src/libfetchers/input-accessor.cc
+++ b/src/libfetchers/input-accessor.cc
@@ -227,6 +227,12 @@ std::ostream & operator << (std::ostream & str, const SourcePath & path)
     return str;
 }
 
+SourcePath SourcePath::append(std::string_view s) const
+{
+    // FIXME: canonicalize?
+    return {accessor, path + s};
+}
+
 struct MemoryInputAccessorImpl : MemoryInputAccessor
 {
     std::map<Path, std::string> files;
diff --git a/src/libfetchers/input-accessor.hh b/src/libfetchers/input-accessor.hh
index 129c1f345..93e891ac2 100644
--- a/src/libfetchers/input-accessor.hh
+++ b/src/libfetchers/input-accessor.hh
@@ -92,6 +92,8 @@ struct SourcePath
     { return accessor.dumpPath(path, sink, filter); }
 
     std::string to_string() const;
+
+    SourcePath append(std::string_view s) const;
 };
 
 std::ostream & operator << (std::ostream & str, const SourcePath & path);