diff --git a/vlib/builtin/builtin.v b/vlib/builtin/builtin.v
index 4d2f8c1987..d88e7d0684 100644
--- a/vlib/builtin/builtin.v
+++ b/vlib/builtin/builtin.v
@@ -170,3 +170,10 @@ pub fn is_atty(fd int) int {
 		return C.isatty(fd)
 	}
 }
+
+fn __as_cast(obj voidptr, obj_type, expected_type int) voidptr {
+	if obj_type != expected_type {
+		panic('as cast: cannot cast $obj_type to $expected_type')
+	}
+	return obj
+}
diff --git a/vlib/v/gen/cgen.v b/vlib/v/gen/cgen.v
index 32017a07fe..db5d274b51 100644
--- a/vlib/v/gen/cgen.v
+++ b/vlib/v/gen/cgen.v
@@ -945,13 +945,7 @@ fn (mut g Gen) expr(node ast.Expr) {
 			}
 		}
 		ast.AsCast {
-			styp := g.typ(it.typ)
-			expr_type_sym := g.table.get_type_symbol(it.expr_type)
-			if expr_type_sym.kind == .sum_type {
-				g.write('/* as */ *($styp*)')
-				g.expr(it.expr)
-				g.write('.obj')
-			}
+			g.as_cast(it)
 		}
 		ast.AssignExpr {
 			g.assign_expr(it)
@@ -1396,7 +1390,8 @@ fn (mut g Gen) infix_expr(node ast.InfixExpr) {
 			g.expr_with_cast(node.right, node.right_type, info.elem_type)
 			g.write(' })')
 		}
-	} else if (node.left_type == node.right_type) && table.is_float(node.left_type) && node.op in [.eq, .ne] {
+	} else if (node.left_type == node.right_type) && table.is_float(node.left_type) && node.op in
+		[.eq, .ne] {
 		// floats should be compared with epsilon
 		if node.left_type == table.f64_type_idx {
 			if node.op == .eq {
@@ -2911,6 +2906,26 @@ fn (mut g Gen) go_stmt(node ast.GoStmt) {
 	}
 }
 
+fn (mut g Gen) as_cast(node ast.AsCast) {
+	// Make sure the sum type can be cast to this type (the types
+	// are the same), otherwise panic.
+	// g.insert_before('
+	styp := g.typ(node.typ)
+	expr_type_sym := g.table.get_type_symbol(node.expr_type)
+	if expr_type_sym.kind == .sum_type {
+		g.write('/* as */ *($styp*)')
+		g.expr(node.expr)
+		g.write('.obj')
+		/*
+		g.write('/* as */ *($styp*)__as_cast(')
+		g.expr(node.expr)
+		g.write('.obj, ')
+		g.expr(node.expr)
+		g.write('.typ, /*expected:*/$node.typ)')
+*/
+	}
+}
+
 fn (mut g Gen) is_expr(node ast.InfixExpr) {
 	g.expr(node.left)
 	g.write('.typ == ')
diff --git a/vlib/v/tests/inout/bad_st_as.out b/vlib/v/tests/inout/bad_st_as.out
new file mode 100644
index 0000000000..f3355d49b5
--- /dev/null
+++ b/vlib/v/tests/inout/bad_st_as.out
@@ -0,0 +1,2 @@
+Foo
+V panic: as cast: cannot cast
diff --git a/vlib/v/tests/inout/bad_st_as.vv b/vlib/v/tests/inout/bad_st_as.vv
new file mode 100644
index 0000000000..d099b15b55
--- /dev/null
+++ b/vlib/v/tests/inout/bad_st_as.vv
@@ -0,0 +1,13 @@
+struct Struct {struct_name string }
+struct Interface {interface_name string}
+
+type Info = Struct | Interface
+
+fn main() {
+	mut info := Info{}
+	info = Struct{struct_name: 'Foo'}
+	s := info as Struct
+	println(s.struct_name)
+	i := info as Interface // wrong
+	println(i.interface_name)
+}