diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 8921ed711a..bc82b9469d 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -3416,9 +3416,14 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 			if enum_decl := c.table.enum_decls[to_sym.name] {
 				mut in_range := false
 				if enum_decl.is_flag {
-					// if a flag enum has 4 variants, the maximum possible value would have all 4 flags set (0b1111)
-					max_val := (u64(1) << enum_decl.fields.len) - 1
-					in_range = node_val >= 0 && u64(node_val) <= max_val
+					if enum_decl.fields.len == 64 {
+						// for 64 fields, just use max_u64 and avoid UB:
+						in_range = node_val >= 0 && u64(node_val) <= max_u64
+					} else {
+						// if a flag enum has 4 variants, the maximum possible value would have all 4 flags set (0b1111)
+						max_val := (u64(1) << enum_decl.fields.len) - 1
+						in_range = node_val >= 0 && u64(node_val) <= max_val
+					}
 				} else {
 					mut enum_val := i64(0)