What it means
The contract called an internal function variable that was never set. This is purely a contract bug: no user input causes it.
How to fix it
Report the transaction hash to the protocol. There is no user-side fix.
All errors
Solidity panics
“panic code 0x51”
Also appears as: “Panic(81)”, “uninitialized function pointer”
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For the full walkthrough of why transactions fail, read the complete error guide.