diff --git a/vlib/v/gen/cgen.v b/vlib/v/gen/cgen.v
index 1274e89b46..c30f3f3694 100644
--- a/vlib/v/gen/cgen.v
+++ b/vlib/v/gen/cgen.v
@@ -325,7 +325,7 @@ fn (g &Gen) base_type(t table.Type) string {
 
 fn (mut g Gen) register_optional(t table.Type, styp string) {
 	// g.typedefs2.writeln('typedef Option $x;')
-	no_ptr := styp.replace('*','_ptr')
+	no_ptr := styp.replace('*', '_ptr')
 	g.hotcode_definitions.writeln('typedef struct {
 		$styp  data;
 		string error;
@@ -3260,7 +3260,12 @@ fn (mut g Gen) is_expr(node ast.InfixExpr) {
 	} else {
 		g.write('.')
 	}
-	g.write('typ == ')
+	sym := g.table.get_type_symbol(node.left_type)
+	if sym.kind == .interface_ {
+		g.write('_interface_idx == ')
+	} else if sym.kind == .sum_type {
+		g.write('typ == ')
+	}
 	g.expr(node.right)
 }
 
diff --git a/vlib/v/tests/interface_test.v b/vlib/v/tests/interface_test.v
index 68391a7d2e..0480a8d4cc 100644
--- a/vlib/v/tests/interface_test.v
+++ b/vlib/v/tests/interface_test.v
@@ -26,7 +26,7 @@ fn (c Cat) name_detailed(pet_name string) string {
 	return '$pet_name the ${typeof(c)}, breed:${c.breed}'
 }
 
-fn (c mut Cat) set_breed(new string) {
+fn (mut c Cat) set_breed(new string) {
 	c.breed = new
 }
 
@@ -49,12 +49,11 @@ fn (d Dog) name_detailed(pet_name string) string {
 	return '$pet_name the ${typeof(d)}, breed:${d.breed}'
 }
 
-fn (d mut Dog) set_breed(new string) {
+fn (mut d Dog) set_breed(new string) {
 	println('Nah')
 }
 
 // do not add to Dog the utility function 'str', so the default one will be used, as a sample
-
 fn test_todo() {
 	if true {
 	} else {
@@ -66,9 +65,9 @@ fn perform_speak(a Animal) {
 	assert true
 	name := a.name()
 	assert name == 'Dog' || name == 'Cat'
-	// if a is Dog {
-	// assert name == 'Dog'
-	// }
+	if a is Dog {
+		assert name == 'Dog'
+	}
 	println(a.name())
 	println('Got animal of type: ${typeof(a)}') // TODO: get implementation type (if possible)
 	// assert a is Dog || a is Cat // TODO: enable when available
@@ -151,7 +150,7 @@ fn test_perform_name_detailed() {
 	perform_speak(cat_persian2)
 	cat_persian2_str := cat_persian2.str()
 	println("Persian Cat 2: '$cat_persian2_str' ...")
-	assert cat_persian2_str == "Custom string conversion for Cat: Persian"
+	assert cat_persian2_str == 'Custom string conversion for Cat: Persian'
 	println('Test (dummy/empty) on array of animals ...')
 	handle_animals([dog, cat])
 	handle_animals_mutable([dog, cat])