diff --git a/src/libstore/posix-fs-canonicalise.cc b/src/libstore/posix-fs-canonicalise.cc
index 2484d51a6..a889938c9 100644
--- a/src/libstore/posix-fs-canonicalise.cc
+++ b/src/libstore/posix-fs-canonicalise.cc
@@ -21,9 +21,9 @@ static void canonicaliseTimestampAndPermissions(const Path & path, const struct
 
         /* Mask out all type related bits. */
         mode_t mode = st.st_mode & ~S_IFMT;
-
-        if (mode != 0444 && mode != 0555) {
-            mode = (st.st_mode & S_IFMT) | 0444 | (st.st_mode & S_IXUSR ? 0111 : 0);
+        bool isDir = S_ISDIR(st.st_mode);
+        if ((mode != 0444 || isDir) && mode != 0555) {
+            mode = (st.st_mode & S_IFMT) | 0444 | (st.st_mode & S_IXUSR || isDir ? 0111 : 0);
             if (chmod(path.c_str(), mode) == -1)
                 throw SysError("changing mode of '%1%' to %2$o", path, mode);
         }