diff --git a/lib/modules.nix b/lib/modules.nix
index 412c7f1df71..de6fadbcb91 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -117,7 +117,7 @@ rec {
         if config._module.check && config._module.freeformType == null && merged.unmatchedDefns != [] then
           let
             firstDef = head merged.unmatchedDefns;
-            baseMsg = "The option `${showOption (prefix ++ firstDef.prefix)}' defined in `${firstDef.file}' does not exist.";
+            baseMsg = "The option `${showOption (prefix ++ firstDef.prefix)}' does not exist. Definition values:${showDefs [ firstDef ]}";
           in
             if attrNames options == [ "_module" ]
               then throw ''
@@ -449,7 +449,7 @@ rec {
       # Handle properties, check types, and merge everything together.
       res =
         if opt.readOnly or false && length defs' > 1 then
-          throw "The option `${showOption loc}' is read-only, but it's set multiple times."
+          throw "The option `${showOption loc}' is read-only, but it's set multiple times. Definition values:${showDefs defs'}"
         else
           mergeDefinitions loc opt.type defs';
 
@@ -497,8 +497,8 @@ rec {
     mergedValue =
       if isDefined then
         if all (def: type.check def.value) defsFinal then type.merge loc defsFinal
-        else let firstInvalid = findFirst (def: ! type.check def.value) null defsFinal;
-        in throw "The option value `${showOption loc}' in `${firstInvalid.file}' is not of type `${type.description}'."
+        else let allInvalid = filter (def: ! type.check def.value) defsFinal;
+        in throw "A definition for option `${showOption loc}' is not of type `${type.description}'. Definition values:${showDefs allInvalid}"
       else
         # (nixos-option detects this specific error message and gives it special
         # handling.  If changed here, please change it there too.)
diff --git a/lib/options.nix b/lib/options.nix
index 73856573a5d..5b7482c8093 100644
--- a/lib/options.nix
+++ b/lib/options.nix
@@ -96,12 +96,12 @@ rec {
     else if all isBool list then foldl' lib.or false list
     else if all isString list then lib.concatStrings list
     else if all isInt list && all (x: x == head list) list then head list
-    else throw "Cannot merge definitions of `${showOption loc}' given in ${showFiles (getFiles defs)}.";
+    else throw "Cannot merge definitions of `${showOption loc}'. Definition values:${showDefs defs}";
 
   mergeOneOption = loc: defs:
     if defs == [] then abort "This case should never happen."
     else if length defs != 1 then
-      throw "The unique option `${showOption loc}' is defined multiple times, in:\n - ${concatStringsSep "\n - " (getFiles defs)}."
+      throw "The unique option `${showOption loc}' is defined multiple times. Definition values:${showDefs defs}"
     else (head defs).value;
 
   /* "Merge" option definitions by checking that they all have the same value. */
@@ -111,11 +111,11 @@ rec {
     # This also makes it work for functions, because the foldl' below would try
     # to compare the first element with itself, which is false for functions
     else if length defs == 1 then (elemAt defs 0).value
-    else foldl' (val: def:
-      if def.value != val then
-        throw "The option `${showOption loc}' has conflicting definitions, in ${showFiles (getFiles defs)}."
+    else (foldl' (first: def:
+      if def.value != first.value then
+        throw "The option `${showOption loc}' has conflicting definition values:${showDefs [ first def ]}"
       else
-        val) (head defs).value defs;
+        first) (head defs) defs).value;
 
   /* Extracts values of all "value" keys of the given list.