diff --git a/src/core/thread/network_diagnostic_tlvs.hpp b/src/core/thread/network_diagnostic_tlvs.hpp
index c7689afd2..d243e35b7 100644
--- a/src/core/thread/network_diagnostic_tlvs.hpp
+++ b/src/core/thread/network_diagnostic_tlvs.hpp
@@ -1044,7 +1044,7 @@ public:
 
 private:
     static constexpr uint16_t kIsLastFlag = 1 << 15;
-    static constexpr uint16_t kIndexMask  = 0x7f;
+    static constexpr uint16_t kIndexMask  = 0x7fff;
 
     uint16_t GetFlagsIndex(void) const { return BigEndian::HostSwap16(mFlagsIndex); }
     void     SetFlagsIndex(uint16_t aFlagsIndex) { mFlagsIndex = BigEndian::HostSwap16(aFlagsIndex); }