diff --git a/lib/generators.nix b/lib/generators.nix
index a71654bec6c3..a64e94bd5cbd 100644
--- a/lib/generators.nix
+++ b/lib/generators.nix
@@ -46,7 +46,10 @@ rec {
     else if isList     v then err "lists" v
     # same as for lists, might want to replace
     else if isAttrs    v then err "attrsets" v
+    # functions can’t be printed of course
     else if isFunction v then err "functions" v
+    # let’s not talk about floats. There is no sensible `toString` for them.
+    else if isFloat    v then err "floats" v
     else err "this value is" (toString v);