diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 18a7401c08cdb3..465abd4dd1784d 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -614,6 +614,16 @@ and use a reference to the sum type instead: `var := &${node.name}(${variant_nam
 				}
 			}
 		} else if sym.info is ast.FnType {
+			if sym.info.func.generic_names.len > 0 {
+				if !variant.typ.has_flag(.generic) {
+					c.error('generic fntype `${sym.name}` must specify generic type names, e.g. ${sym.name}[T]',
+						variant.pos)
+				}
+				if node.generic_types.len == 0 {
+					c.error('generic sumtype `${node.name}` must specify generic type names, e.g. ${node.name}[T]',
+						node.name_pos)
+				}
+			}
 			if c.table.sym(sym.info.func.return_type).name.ends_with('.${node.name}') {
 				c.error('sum type `${node.name}` cannot be defined recursively', variant.pos)
 			}
@@ -2889,6 +2899,11 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 	if to_type.has_flag(.option) && from_type == ast.none_type {
 		// allow conversion from none to every option type
 	} else if to_sym.kind == .sum_type {
+		to_sym_info := to_sym.info as ast.SumType
+		if to_sym_info.generic_types.len > 0 && to_sym_info.concrete_types.len == 0 {
+			c.error('generic sumtype `${to_sym.name}` must specify type parameter, e.g. ${to_sym.name}[int]',
+				node.pos)
+		}
 		if from_type in [ast.int_literal_type, ast.float_literal_type] {
 			xx := if from_type == ast.int_literal_type { ast.int_type } else { ast.f64_type }
 			node.expr_type = c.promote_num(node.expr_type, xx)
diff --git a/vlib/v/checker/tests/generic_sumtype_decl_err_d.out b/vlib/v/checker/tests/generic_sumtype_decl_err_d.out
new file mode 100644
index 00000000000000..a748a26e068f31
--- /dev/null
+++ b/vlib/v/checker/tests/generic_sumtype_decl_err_d.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/generic_sumtype_decl_err_d.vv:5:15: error: generic fntype `Fn` must specify generic type names, e.g. Fn[T]
+    3 | type Fnn = fn (string) string
+    4 |
+    5 | type Parser = Fn | Fnn
+      |               ~~
+    6 |
+    7 | fn main() {
diff --git a/vlib/v/checker/tests/generic_sumtype_decl_err_d.vv b/vlib/v/checker/tests/generic_sumtype_decl_err_d.vv
new file mode 100644
index 00000000000000..d1a870cb63935a
--- /dev/null
+++ b/vlib/v/checker/tests/generic_sumtype_decl_err_d.vv
@@ -0,0 +1,9 @@
+type Fn[T] = fn (string) T
+
+type Fnn = fn (string) string
+
+type Parser = Fn | Fnn
+
+fn main() {
+	println('hello')
+}
diff --git a/vlib/v/checker/tests/generic_sumtype_decl_err_e.out b/vlib/v/checker/tests/generic_sumtype_decl_err_e.out
new file mode 100644
index 00000000000000..be4ac16fa15b0b
--- /dev/null
+++ b/vlib/v/checker/tests/generic_sumtype_decl_err_e.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/generic_sumtype_decl_err_e.vv:10:10: error: generic sumtype `Parser` must specify type parameter, e.g. Parser[int]
+    8 |
+    9 | fn main() {
+   10 |     _ := Parser(f)
+      |          ~~~~~~~~~
+   11 | }
diff --git a/vlib/v/checker/tests/generic_sumtype_decl_err_e.vv b/vlib/v/checker/tests/generic_sumtype_decl_err_e.vv
new file mode 100644
index 00000000000000..362ff7dcb43886
--- /dev/null
+++ b/vlib/v/checker/tests/generic_sumtype_decl_err_e.vv
@@ -0,0 +1,11 @@
+type Fn[T] = fn (string) T
+
+type Fnn = fn (string) string
+
+type Parser[T] = Fn[T] | Fnn
+
+fn f(x string) string { return '>> f $x' }
+
+fn main() {
+    _ := Parser(f)
+}