diff --git a/code-generator/src/main/resources/io/streamnative/lightproto/generator/LightProtoCodec.java b/code-generator/src/main/resources/io/streamnative/lightproto/generator/LightProtoCodec.java
index 2a081d5..619030a 100644
--- a/code-generator/src/main/resources/io/streamnative/lightproto/generator/LightProtoCodec.java
+++ b/code-generator/src/main/resources/io/streamnative/lightproto/generator/LightProtoCodec.java
@@ -425,8 +425,24 @@ static int writeRawVarInt64(byte[] a, int i, long value) {
         }
     }
 
+    /**
+     * Writes n as an unsigned 32-bit varint, at most 5 bytes, as protobuf encodes
+     * zigzag-encoded sint32 values: what computeVarUIntSize() counts.
+     * writeRawVarInt() is the int32 encoding, which writes a negative n as 10 bytes.
+     */
+    static int writeRawVarUInt(byte[] a, int i, int n) {
+        while (true) {
+            if ((n & ~0x7F) == 0) {
+                a[i++] = (byte) n;
+                return i;
+            }
+            a[i++] = (byte) ((n & 0x7F) | 0x80);
+            n >>>= 7;
+        }
+    }
+
     static int writeRawSignedVarInt(byte[] a, int i, int n) {
-        return writeRawVarInt(a, i, encodeZigZag32(n));
+        return writeRawVarUInt(a, i, encodeZigZag32(n));
     }
 
     static int writeRawSignedVarInt64(byte[] a, int i, long n) {
@@ -513,8 +529,19 @@ static int writeRawVarInt64(java.nio.ByteBuffer nb, int i, long value) {
         }
     }
 
+    static int writeRawVarUInt(java.nio.ByteBuffer nb, int i, int n) {
+        while (true) {
+            if ((n & ~0x7F) == 0) {
+                nb.put(i++, (byte) n);
+                return i;
+            }
+            nb.put(i++, (byte) ((n & 0x7F) | 0x80));
+            n >>>= 7;
+        }
+    }
+
     static int writeRawSignedVarInt(java.nio.ByteBuffer nb, int i, int n) {
-        return writeRawVarInt(nb, i, encodeZigZag32(n));
+        return writeRawVarUInt(nb, i, encodeZigZag32(n));
     }
 
     static int writeRawSignedVarInt64(java.nio.ByteBuffer nb, int i, long n) {
diff --git a/tests/src/test/java/io/streamnative/lightproto/tests/NumbersTest.java b/tests/src/test/java/io/streamnative/lightproto/tests/NumbersTest.java
index b2dfb4d..e130f4d 100644
--- a/tests/src/test/java/io/streamnative/lightproto/tests/NumbersTest.java
+++ b/tests/src/test/java/io/streamnative/lightproto/tests/NumbersTest.java
@@ -20,6 +20,8 @@
 import io.netty.buffer.Unpooled;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import java.util.Arrays;
 
@@ -204,6 +206,13 @@ public void testNumberFields() throws Exception {
         verify(lpn, pbn.build());
     }
 
+    @ParameterizedTest
+    @ValueSource(ints = {1 << 30, -(1 << 30) - 1, Integer.MAX_VALUE, Integer.MIN_VALUE})
+    public void testSint32WithZigZagTopBitSet(int value) throws Exception {
+        // The zigzag encoding of these values is >= 2^31 unsigned: still a 5-byte varint
+        verify(new Numbers().setXSint32(value), NumbersOuterClass.Numbers.newBuilder().setXSint32(value).build());
+    }
+
     @Test
     public void testClearResetsAllFieldsToDefaults() throws Exception {
         Numbers lpn = new Numbers();
diff --git a/tests/src/test/java/io/streamnative/lightproto/tests/RepeatedNumbersTest.java b/tests/src/test/java/io/streamnative/lightproto/tests/RepeatedNumbersTest.java
index 791d08e..08d8ff0 100644
--- a/tests/src/test/java/io/streamnative/lightproto/tests/RepeatedNumbersTest.java
+++ b/tests/src/test/java/io/streamnative/lightproto/tests/RepeatedNumbersTest.java
@@ -485,4 +485,22 @@ public void testNumberFieldsPacked() throws Exception {
         assertEquals(-12L, parsed.getXSint64At(0));
         assertEquals(-13L, parsed.getXSint64At(1));
     }
+
+    @Test
+    public void testSint32WithZigZagTopBitSet() throws Exception {
+        // The zigzag encoding of these values is >= 2^31 unsigned: still a 5-byte varint
+        int[] values = {1 << 30, -(1 << 30) - 1, Integer.MAX_VALUE, Integer.MIN_VALUE};
+        Repeated lp = new Repeated();
+        RepeatedPacked lpPacked = new RepeatedPacked();
+        RepeatedNumbers.Repeated.Builder pb = RepeatedNumbers.Repeated.newBuilder();
+        RepeatedNumbers.RepeatedPacked.Builder pbPacked = RepeatedNumbers.RepeatedPacked.newBuilder();
+        for (int value : values) {
+            lp.addXSint32(value);
+            lpPacked.addXSint32(value);
+            pb.addXSint32(value);
+            pbPacked.addXSint32(value);
+        }
+        assertArrayEquals(pb.build().toByteArray(), lp.toByteArray());
+        assertArrayEquals(pbPacked.build().toByteArray(), lpPacked.toByteArray());
+    }
 }