diff --git a/protobuf_serialization.nim b/protobuf_serialization.nim
index 2b9fbdd..2bfd54c 100644
--- a/protobuf_serialization.nim
+++ b/protobuf_serialization.nim
@@ -173,7 +173,7 @@ proc decodeField*[T: SomeVarint](
   numBytesToRead = none(int)
 ): ProtoField[T] {.inline.} =
   # Only up to 128 bits supported by the spec
-  assert (bytes.len - 1) <= 16
+  assert sizeof(T) <= 16
 
   var bytesRead = 0
 
diff --git a/tests/test_serialization.nim b/tests/test_serialization.nim
index b6b2051..16278d4 100644
--- a/tests/test_serialization.nim
+++ b/tests/test_serialization.nim
@@ -8,11 +8,13 @@ type
 type
   Test1 = object
     a: uint
+    b: string
 
   Test3 = object
     g {.sfixed32.}: int
     h: int
     i: Test1
+    j: string
 
 suite "Test Varint Encoding":
   test "Can encode/decode enum field":
@@ -83,7 +85,7 @@ suite "Test Varint Encoding":
   test "Can encode/decode object field":
     var proto = newProtoBuffer()
 
-    let obj = Test3(g: 300, h: 200, i: Test1(a: 100))
+    let obj = Test3(g: 300, h: 200, i: Test1(a: 100, b: "this is a test"), j: "testing")
 
     proto.encodeField(obj)
     var offset, bytesProcessed: int
@@ -96,7 +98,7 @@ suite "Test Varint Encoding":
   test "Can encode/decode object":
     var proto = newProtoBuffer()
 
-    let obj = Test3(g: 300, h: 200, i: Test1(a: 100))
+    let obj = Test3(g: 300, h: 200, i: Test1(a: 100, b: "this is a test"), j: "testing")
 
     proto.encode(obj)
     var output = proto.output