diff --git a/vlib/v/gen/c/fn.v b/vlib/v/gen/c/fn.v
index 1e7d9a364c..b91dfa1e26 100644
--- a/vlib/v/gen/c/fn.v
+++ b/vlib/v/gen/c/fn.v
@@ -1471,7 +1471,37 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 	} else {
 		node.args
 	}
-	expected_types := node.expected_arg_types
+	mut expected_types := node.expected_arg_types
+	// unwrap generics fn/method arguments to concretes
+	if node.concrete_types.len > 0 && node.concrete_types.all(!it.has_flag(.generic)) {
+		if node.is_method {
+			if func := g.table.find_method(g.table.sym(node.left_type), node.name) {
+				if func.generic_names.len > 0 {
+					for i in 0 .. expected_types.len {
+						mut muttable := unsafe { &ast.Table(g.table) }
+						if utyp := muttable.resolve_generic_to_concrete(node.expected_arg_types[i],
+							func.generic_names, node.concrete_types)
+						{
+							expected_types[i] = utyp
+						}
+					}
+				}
+			}
+		} else {
+			if func := g.table.find_fn(node.name) {
+				if func.generic_names.len > 0 {
+					for i in 0 .. expected_types.len {
+						mut muttable := unsafe { &ast.Table(g.table) }
+						if utyp := muttable.resolve_generic_to_concrete(node.expected_arg_types[i],
+							func.generic_names, node.concrete_types)
+						{
+							expected_types[i] = utyp
+						}
+					}
+				}
+			}
+		}
+	}
 	// only v variadic, C variadic args will be appeneded like normal args
 	is_variadic := expected_types.len > 0 && expected_types.last().has_flag(.variadic)
 		&& node.language == .v
diff --git a/vlib/v/tests/generics_method_with_sumtype_args_test.v b/vlib/v/tests/generics_method_with_sumtype_args_test.v
new file mode 100644
index 0000000000..3ff6fc0b79
--- /dev/null
+++ b/vlib/v/tests/generics_method_with_sumtype_args_test.v
@@ -0,0 +1,26 @@
+module main
+
+fn test_generics_method_with_sumtype_args() {
+	mut me := CatDad{}
+	ret := me.adopt<CatType, Cat>(CatType.black, BlackCat{})
+	println(ret)
+	assert ret == 22
+}
+
+[flag]
+enum CatType {
+	black
+	white
+}
+
+type Cat = BlackCat | WhiteCat
+
+struct BlackCat {}
+
+struct WhiteCat {}
+
+struct CatDad {}
+
+fn (mut foo CatDad) adopt<D, T>(d D, t T) int {
+	return 22
+}