diff --git a/vlib/v/fmt/tests/anon_fn_as_param_keep.vv b/vlib/v/fmt/tests/anon_fn_as_param_keep.vv
index dd5f3180ea..93a97e736c 100644
--- a/vlib/v/fmt/tests/anon_fn_as_param_keep.vv
+++ b/vlib/v/fmt/tests/anon_fn_as_param_keep.vv
@@ -1,4 +1,4 @@
-pub fn (a []int) reduce(iter fn (int, int) int, accum_start int) int {
+pub fn reduce(a []int, iter fn (int, int) int, accum_start int) int {
 	iter(accum_start)
 }
 
diff --git a/vlib/v/parser/fn.v b/vlib/v/parser/fn.v
index 3cb57d84fa..9cf036e8d1 100644
--- a/vlib/v/parser/fn.v
+++ b/vlib/v/parser/fn.v
@@ -281,7 +281,14 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		// Do not allow to modify / add methods to types from other modules
 		// arrays/maps dont belong to a module only their element types do
 		// we could also check if kind is .array,  .array_fixed, .map instead of mod.len
-		if type_sym.mod.len > 0 && type_sym.mod != p.mod && type_sym.language == .v {
+		mut is_non_local := type_sym.mod.len > 0 && type_sym.mod != p.mod && type_sym.language == .v
+		// check maps & arrays, must be defined in same module as the elem type
+		if !is_non_local && type_sym.kind in [.array, .map] {
+			elem_type_sym := p.table.get_type_symbol(p.table.value_type(rec_type))
+			is_non_local = elem_type_sym.mod.len > 0 &&
+				elem_type_sym.mod != p.mod && elem_type_sym.language == .v
+		}
+		if is_non_local {
 			p.error_with_pos('cannot define new methods on non-local type $type_sym.name',
 				rec_type_pos)
 			return ast.FnDecl{}
diff --git a/vlib/v/parser/tests/method_decl_on_non_local_array.out b/vlib/v/parser/tests/method_decl_on_non_local_array.out
new file mode 100644
index 0000000000..f01fc00045
--- /dev/null
+++ b/vlib/v/parser/tests/method_decl_on_non_local_array.out
@@ -0,0 +1,5 @@
+vlib/v/parser/tests/method_decl_on_non_local_array.vv:1:7: error: cannot define new methods on non-local type []int
+    1 | fn (a []int) get_number() int { 
+      |       ~~~~~
+    2 |     return 1
+    3 | }
diff --git a/vlib/v/parser/tests/method_decl_on_non_local_array.vv b/vlib/v/parser/tests/method_decl_on_non_local_array.vv
new file mode 100644
index 0000000000..9822ce51a8
--- /dev/null
+++ b/vlib/v/parser/tests/method_decl_on_non_local_array.vv
@@ -0,0 +1,3 @@
+fn (a []int) get_number() int { 
+	return 1
+}
diff --git a/vlib/v/parser/tests/method_decl_on_non_local_map.out b/vlib/v/parser/tests/method_decl_on_non_local_map.out
new file mode 100644
index 0000000000..d8459da3a3
--- /dev/null
+++ b/vlib/v/parser/tests/method_decl_on_non_local_map.out
@@ -0,0 +1,5 @@
+vlib/v/parser/tests/method_decl_on_non_local_map.vv:1:7: error: cannot define new methods on non-local type map[string]string
+    1 | fn (a map[string]string) get_number() int { 
+      |       ~~~~~~~~~~~~~~~~~
+    2 |     return 1
+    3 | }
diff --git a/vlib/v/parser/tests/method_decl_on_non_local_map.vv b/vlib/v/parser/tests/method_decl_on_non_local_map.vv
new file mode 100644
index 0000000000..f95e719f2d
--- /dev/null
+++ b/vlib/v/parser/tests/method_decl_on_non_local_map.vv
@@ -0,0 +1,3 @@
+fn (a map[string]string) get_number() int { 
+	return 1
+}
diff --git a/vlib/v/parser/tests/method_decl_on_non_local_type.out b/vlib/v/parser/tests/method_decl_on_non_local_type.out
new file mode 100644
index 0000000000..1b57bbd2b0
--- /dev/null
+++ b/vlib/v/parser/tests/method_decl_on_non_local_type.out
@@ -0,0 +1,5 @@
+vlib/v/parser/tests/method_decl_on_non_local_type.vv:1:7: error: cannot define new methods on non-local type int
+    1 | fn (a int) get_number() int { 
+      |       ~~~
+    2 |     return 1
+    3 | }
diff --git a/vlib/v/parser/tests/method_decl_on_non_local_type.vv b/vlib/v/parser/tests/method_decl_on_non_local_type.vv
new file mode 100644
index 0000000000..f75565bc71
--- /dev/null
+++ b/vlib/v/parser/tests/method_decl_on_non_local_type.vv
@@ -0,0 +1,3 @@
+fn (a int) get_number() int { 
+	return 1
+}