Solana Non-base58 character

I'm trying to sign a transaction on solana with this piece of code:

                const txs = [];

                const { recentBlockhash, instructions, feePayer } = tx;

                const newTx = new SolTransaction({
                    feePayer,
                    recentBlockhash,
                });
                
                for (const ins of instructions) {
                    newTx.add(ins);
                }
                

                newTx.sign({ publicKey: this._signer.publicKey, secretKey: this._signer.secretKey });
               

                txs.push(newTx);

but for some reason when i sign the transaction i get this error: Non-base58 character any idea why?