diff --git a/packages/protons-runtime/src/utils/longbits.ts b/packages/protons-runtime/src/utils/longbits.ts
index 12cc5d4..31b120f 100644
--- a/packages/protons-runtime/src/utils/longbits.ts
+++ b/packages/protons-runtime/src/utils/longbits.ts
@@ -22,12 +22,12 @@ export class LongBits {
     /**
      * Low bits
      */
-    this.lo = lo | 0
+    this.lo = lo >>> 0
 
     /**
      * High bits
      */
-    this.hi = hi | 0
+    this.hi = hi >>> 0
   }
 
   /**
diff --git a/packages/protons/test/longbits.spec.ts b/packages/protons/test/longbits.spec.ts
new file mode 100644
index 0000000..26b78f8
--- /dev/null
+++ b/packages/protons/test/longbits.spec.ts
@@ -0,0 +1,24 @@
+import { expect } from 'aegir/chai'
+import { writer } from 'protons-runtime'
+
+// low word in [2^31, 2^32) used to be stored signed and written as one byte
+describe('64-bit varints', () => {
+  const cases: Array<[bigint, number[]]> = [
+    [2147483647n, [0xff, 0xff, 0xff, 0xff, 0x07]],
+    [2147483648n, [0x80, 0x80, 0x80, 0x80, 0x08]],
+    [3221225472n, [0x80, 0x80, 0x80, 0x80, 0x0c]],
+    [4294967295n, [0xff, 0xff, 0xff, 0xff, 0x0f]],
+    [4294967296n, [0x80, 0x80, 0x80, 0x80, 0x10]]
+  ]
+
+  for (const method of ['uint64', 'int64'] as const) {
+    for (const [value, bytes] of cases) {
+      it(`should encode ${method} ${value}`, () => {
+        const w = writer()
+        w[method](value)
+
+        expect(w.finish()).to.equalBytes(bytes)
+      })
+    }
+  }
+})