diff --git a/lib/modules.nix b/lib/modules.nix
index 33a0d84a6d7..d3f10944e70 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -361,6 +361,17 @@ rec {
       */
       byName = attr: f: modules:
         foldl' (acc: module:
+              if !(builtins.isAttrs module.${attr}) then
+                throw ''
+                  You're trying to declare a value of type `${builtins.typeOf module.${attr}}'
+                  rather than an attribute-set for the option
+                  `${builtins.concatStringsSep "." prefix}'!
+
+                  This usually happens if `${builtins.concatStringsSep "." prefix}' has option
+                  definitions inside that are not matched. Please check how to properly define
+                  this option by e.g. referring to `man 5 configuration.nix'!
+                ''
+              else
                 acc // (mapAttrs (n: v:
                                    (acc.${n} or []) ++ f module v
                                  ) module.${attr}
diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index f843d303e44..2eddeec07b1 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -169,7 +169,7 @@ checkConfigOutput "foo" config.submodule.foo ./declare-submoduleWith-special.nix
 ## shorthandOnlyDefines config behaves as expected
 checkConfigOutput "true" config.submodule.config ./declare-submoduleWith-shorthand.nix ./define-submoduleWith-shorthand.nix
 checkConfigError 'is not of type `boolean' config.submodule.config ./declare-submoduleWith-shorthand.nix ./define-submoduleWith-noshorthand.nix
-checkConfigError 'value is a boolean while a set was expected' config.submodule.config ./declare-submoduleWith-noshorthand.nix ./define-submoduleWith-shorthand.nix
+checkConfigError "You're trying to declare a value of type \`bool'\nrather than an attribute-set for the option" config.submodule.config ./declare-submoduleWith-noshorthand.nix ./define-submoduleWith-shorthand.nix
 checkConfigOutput "true" config.submodule.config ./declare-submoduleWith-noshorthand.nix ./define-submoduleWith-noshorthand.nix
 
 ## submoduleWith should merge all modules in one swoop