diff --git a/src/varint.zig b/src/varint.zig
index 7a4420c..8f0a925 100644
--- a/src/varint.zig
+++ b/src/varint.zig
@@ -34,7 +34,7 @@ pub fn uvarint(buf: []const u8) error{Overflow}!Varint {
             return Varint{ .value = x | (@as(u64, b) << s), .bytesRead = i + 1 };
         }
         x |= (@as(u64, b & 0x7f) << s);
-        s += 7;
+        s = try std.math.add(u6, s, 7);
     }
 
     return Varint{
@@ -88,3 +88,17 @@ test "uvarint" {
         try testing.expectEqual(n, got.bytesRead);
     }
 }
+
+const overflowTest = struct {
+    arr: []const u8,
+};
+
+test "overflow" {
+    for ([_][]const u8{
+        &[_]u8{ 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x2 },
+        &[_]u8{ 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x80, 0x1, 0, 0 },
+        &[_]u8{ 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF },
+    }) |t| {
+        try testing.expectError(error.Overflow, uvarint(t));
+    }
+}