diff --git a/vlib/v/fmt/tests/struct_with_fn_fields_keep.vv b/vlib/v/fmt/tests/struct_with_fn_fields_keep.vv
new file mode 100644
index 0000000000..2f1bd98316
--- /dev/null
+++ b/vlib/v/fmt/tests/struct_with_fn_fields_keep.vv
@@ -0,0 +1,4 @@
+struct FieldsWithOptionalVoidReturnType {
+	f fn () ?
+	g fn () ?
+}
diff --git a/vlib/v/table/table.v b/vlib/v/table/table.v
index fa4f65ed64..6535c0cb7c 100644
--- a/vlib/v/table/table.v
+++ b/vlib/v/table/table.v
@@ -120,7 +120,9 @@ pub fn (t &Table) fn_type_source_signature(f &Fn) string {
 		}
 	}
 	sig += ')'
-	if f.return_type != void_type {
+	if f.return_type == ovoid_type {
+		sig += ' ?'
+	} else if f.return_type != void_type {
 		return_type_sym := t.get_type_symbol(f.return_type)
 		sig += ' $return_type_sym.name'
 	}
diff --git a/vlib/v/table/types.v b/vlib/v/table/types.v
index 13b0d16ca5..ce97f1c1d4 100644
--- a/vlib/v/table/types.v
+++ b/vlib/v/table/types.v
@@ -335,6 +335,7 @@ pub const (
 
 pub const (
 	void_type    = new_type(void_type_idx)
+	ovoid_type   = new_type(void_type_idx).set_flag(.optional) // the return type of `fn () ?`
 	voidptr_type = new_type(voidptr_type_idx)
 	byteptr_type = new_type(byteptr_type_idx)
 	charptr_type = new_type(charptr_type_idx)