diff --git a/src/datatypes/compiler-utils.js b/src/datatypes/compiler-utils.js
index d60eda5..6625a51 100644
--- a/src/datatypes/compiler-utils.js
+++ b/src/datatypes/compiler-utils.js
@@ -154,6 +154,7 @@ let val = value._value ${big ? '|| 0n' : ''}
 for (const key in flags) {
   if (value[key]) val |= flags[key]
 }
+${(!big && /^l?u/.test(type)) ? 'val = val >>> 0 // unsigned underlying type: keep bit 31 from making val negative and rejected' : ''}
 return (ctx.${type})(val, buffer, offset)
       `.trim())
     }],
@@ -208,6 +209,7 @@ let val = value._value ${big ? '|| 0n' : ''}
 for (const key in flags) {
   if (value[key]) val |= flags[key]
 }
+${(!big && /^l?u/.test(type)) ? 'val = val >>> 0 // unsigned underlying type (see above)' : ''}
 return (ctx.${type})(val)
       `.trim())
     }],
diff --git a/src/datatypes/utils.js b/src/datatypes/utils.js
index 2f1722b..40adce8 100644
--- a/src/datatypes/utils.js
+++ b/src/datatypes/utils.js
@@ -257,6 +257,10 @@ function writeBitflags (value, buffer, offset, { type, flags, shift, big }, root
   for (const key in f) {
     if (value[key]) val |= f[key]
   }
+  // Coerce to unsigned only for unsigned underlying types: |= is signed in JS, so bit 31 makes val negative and the
+  // unsigned writer rejects it. Signed types (i8/i16/i32/li32) keep the |= result, which is already the correct value;
+  // forcing them unsigned would push a valid negative out of the signed writer's range.
+  if (!big && /^l?u/.test(type)) val = val >>> 0
   return this.write(val, buffer, offset, type, rootNode)
 }
 
diff --git a/test/misc.js b/test/misc.js
index fc726e9..6877ba4 100644
--- a/test/misc.js
+++ b/test/misc.js
@@ -26,6 +26,51 @@ describe('mapper', () => {
   }
 })
 
+describe('bitflags', () => {
+  // A 32-bit bitflags whose top flag is bit 31. `|=` is signed in JS, so building the value makes it negative;
+  // the writer must treat it as unsigned or writeUInt32LE rejects it (regression for a bit-31 write crash).
+  const flags = Array.from({ length: 32 }, (_, i) => (i === 31 ? 'topbit' : 'f' + i))
+  const type = ['bitflags', { type: 'lu32', flags }]
+  const proto = new ProtoDef()
+  proto.addType('flags32', type)
+  const compiler = new ProtoDefCompiler()
+  compiler.addTypesToCompile({ flags32: type })
+  const compiled = compiler.compileProtoDefSync()
+
+  for (const [label, p] of [['interpreted', proto], ['compiled', compiled]]) {
+    it(`round-trips a value with bit 31 set (${label})`, () => {
+      const buf = p.createPacketBuffer('flags32', { topbit: true, f0: true })
+      assert.deepStrictEqual(buf, Buffer.from([0x01, 0x00, 0x00, 0x80]))
+      const back = p.parsePacketBuffer('flags32', buf).data
+      assert.strictEqual(back.topbit, true)
+      assert.strictEqual(back.f0, true)
+      assert.strictEqual(back.f1, false)
+    })
+  }
+})
+
+describe('bitflags with a signed underlying type', () => {
+  // Signed underlying type with bit 31 set: reading 0xffffffff as i32 yields -1. The unsigned coercion must NOT apply
+  // here, or writing the decoded value pushes -1 to 4294967295 and the signed writer rejects it (a regression the
+  // unsigned bit-31 fix introduced). The |= result is already the correct signed value.
+  const type = ['bitflags', { type: 'i32', flags: { top: 31 }, shift: true }]
+  const proto = new ProtoDef()
+  proto.addType('sflags', type)
+  const compiler = new ProtoDefCompiler()
+  compiler.addTypesToCompile({ sflags: type })
+  const compiled = compiler.compileProtoDefSync()
+
+  for (const [label, p] of [['interpreted', proto], ['compiled', compiled]]) {
+    it(`round-trips a signed value with bit 31 set (${label})`, () => {
+      const buf = Buffer.from([0xff, 0xff, 0xff, 0xff]) // i32 -1, top bit set
+      const obj = p.parsePacketBuffer('sflags', buf).data
+      assert.strictEqual(obj.top, true)
+      const back = p.createPacketBuffer('sflags', obj) // must not throw and must reproduce the original bytes
+      assert.deepStrictEqual(back, buf)
+    })
+  }
+})
+
 describe('FullPacketParser', () => {
   const packet = ['container', [{ name: 'a', type: 'i32' }]]
   const proto = new ProtoDef()