diff --git a/src/core/common/encoding.hpp b/src/core/common/encoding.hpp
index bf35678a0..786c8aa4f 100644
--- a/src/core/common/encoding.hpp
+++ b/src/core/common/encoding.hpp
@@ -76,7 +76,7 @@ inline uint32_t Reverse32(uint32_t v)
     return v;
 }
 
-#define BitVectorBytes(x) (((x) + (CHAR_BIT - 1)) / CHAR_BIT)
+#define BitVectorBytes(x) static_cast<uint8_t>(((x) + (CHAR_BIT - 1)) / CHAR_BIT)
 
 namespace BigEndian {
 
diff --git a/src/core/thread/network_data_tlvs.hpp b/src/core/thread/network_data_tlvs.hpp
index 26de34ad2..4de23f10f 100644
--- a/src/core/thread/network_data_tlvs.hpp
+++ b/src/core/thread/network_data_tlvs.hpp
@@ -345,7 +345,8 @@ public:
     bool IsValid(void) const
     {
         return ((GetLength() >= sizeof(*this) - sizeof(NetworkDataTlv)) &&
-                (GetLength() >= BitVectorBytes(mPrefixLength) + sizeof(*this) - sizeof(NetworkDataTlv)));
+                (GetLength() >= BitVectorBytes(mPrefixLength) + sizeof(*this) - sizeof(NetworkDataTlv)) &&
+                (BitVectorBytes(mPrefixLength) <= sizeof(Ip6::Address)));
     }
 
     /**