diff --git a/lib/strings.nix b/lib/strings.nix
index d48624257cf..1cc633c729d 100644
--- a/lib/strings.nix
+++ b/lib/strings.nix
@@ -438,8 +438,13 @@ rec {
        => true
        isStorePath pkgs.python
        => true
+       isStorePath [] || isStorePath 42 || isStorePath {} || …
+       => false
   */
-  isStorePath = x: builtins.substring 0 1 (toString x) == "/" && dirOf (builtins.toPath x) == builtins.storeDir;
+  isStorePath = x:
+       builtins.isString x
+    && builtins.substring 0 1 (toString x) == "/"
+    && dirOf (builtins.toPath x) == builtins.storeDir;
 
   /* Convert string to int
      Obviously, it is a bit hacky to use fromJSON that way.