diff --git a/src/aro/Parser.zig b/src/aro/Parser.zig
index 0b200a34..31fa9c17 100644
--- a/src/aro/Parser.zig
+++ b/src/aro/Parser.zig
@@ -2701,8 +2701,6 @@ const Enumerator = struct {
             return;
         }
         if (try e.res.val.add(e.res.val, Value.one, e.res.ty, p.comp)) {
-            const byte_size = e.res.ty.sizeof(p.comp).?;
-            const bit_size: u8 = @intCast(if (e.res.ty.isUnsignedInt(p.comp)) byte_size * 8 else byte_size * 8 - 1);
             if (e.fixed) {
                 try p.errStr(.enum_not_representable_fixed, tok, try p.typeStr(e.res.ty));
                 return;
@@ -2711,6 +2709,8 @@ const Enumerator = struct {
                 try p.errTok(.enumerator_overflow, tok);
                 break :blk larger;
             } else blk: {
+                const signed = !e.res.ty.isUnsignedInt(p.comp);
+                const bit_size: u8 = @intCast(e.res.ty.bitSizeof(p.comp).? - @intFromBool(signed));
                 try p.errExtra(.enum_not_representable, tok, .{ .pow_2_as_string = bit_size });
                 break :blk Type{ .specifier = .ulong_long };
             };
diff --git a/test/cases/_BitInt.c b/test/cases/_BitInt.c
index 5ad4cced..4f1e83d2 100644
--- a/test/cases/_BitInt.c
+++ b/test/cases/_BitInt.c
@@ -22,6 +22,11 @@ int z = 0uwb;
 int x = 1'2;
 _Static_assert(((int)-18446744073709551616WB) == 0);
 
+enum E: _BitInt(512) {
+    A=6703903964971298549787012499102923063739682910296196688861780721860882015036773488400937149083451713845015929093243025426876941405973284973216824503042047WB,
+    B,
+};
+
 #define EXPECTED_ERRORS "_BitInt.c:3:1: warning: '_BitInt' in C17 and earlier is a Clang extension' [-Wbit-int-extension]" \
     "_BitInt.c:13:1: error: _BitInt of bit sizes greater than 65535 not supported" \
     "_BitInt.c:14:8: error: signed _BitInt must have a bit size of at least 2" \
@@ -30,3 +35,5 @@ _Static_assert(((int)-18446744073709551616WB) == 0);
     "_BitInt.c:18:25: warning: '_BitInt' suffix for literals is a C23 extension [-Wc23-extensions]" \
     "_BitInt.c:18:25: warning: '_BitInt' suffix for literals is a C23 extension [-Wc23-extensions]" \
     "_BitInt.c:22:10: error: expected ';', found 'a character literal'" \
+    "_BitInt.c:27:5: error: enumerator value is not representable in the underlying type '_BitInt'" \
+