diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index a14fe8cb30a5..2a48d88cbf73 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -294,6 +294,9 @@ checkConfigOutput '^"42"$' config.value ./declare-coerced-value.nix
 checkConfigOutput '^"24"$' config.value ./declare-coerced-value.nix ./define-value-string.nix
 checkConfigError 'A definition for option .* is not.*string or signed integer convertible to it.*. Definition values:\n\s*- In .*: \[ \]' config.value ./declare-coerced-value.nix ./define-value-list.nix
 
+# Check coerced option merging.
+checkConfigError 'The option .value. in .*/declare-coerced-value.nix. is already declared in .*/declare-coerced-value-no-default.nix.' config.value ./declare-coerced-value.nix ./declare-coerced-value-no-default.nix
+
 # Check coerced value with unsound coercion
 checkConfigOutput '^12$' config.value ./declare-coerced-value-unsound.nix
 checkConfigError 'A definition for option .* is not of type .*. Definition values:\n\s*- In .*: "1000"' config.value ./declare-coerced-value-unsound.nix ./define-value-string-bigint.nix
diff --git a/lib/tests/modules/declare-coerced-value-no-default.nix b/lib/tests/modules/declare-coerced-value-no-default.nix
new file mode 100644
index 000000000000..820913f920c3
--- /dev/null
+++ b/lib/tests/modules/declare-coerced-value-no-default.nix
@@ -0,0 +1,9 @@
+{ lib, ... }:
+
+{
+  options = {
+    value = lib.mkOption {
+      type = lib.types.coercedTo lib.types.int builtins.toString lib.types.str;
+    };
+  };
+}
diff --git a/lib/types.nix b/lib/types.nix
index 8b1d19ff2c61..86b717afd122 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -1035,7 +1035,7 @@ rec {
         getSubOptions = finalType.getSubOptions;
         getSubModules = finalType.getSubModules;
         substSubModules = m: coercedTo coercedType coerceFunc (finalType.substSubModules m);
-        typeMerge = t1: t2: null;
+        typeMerge = t: null;
         functor = (defaultFunctor name) // { wrapped = finalType; };
         nestedTypes.coercedType = coercedType;
         nestedTypes.finalType = finalType;