A 52-year-old woman with a 10-year history of Type 2 diabetes mellitus presents with recurrent episodes of symptomatic hypoglycemia (tremor, sweating, confusion) despite taking metformin 1000 mg BD and glibenclamide 10 mg daily. Her HbA₁c is 6.8%. Fasting glucose is 95 mg/dL, and a 2-hour postprandial glucose is 140 mg/dL. During a hypoglycemic episode, serum glucose is 52 mg/dL, but insulin level is 8.2 mIU/L and C-peptide is 1.8 ng/mL (both inappropriately low for hypoglycemia). Which of the following best explains the failure of glucagon to prevent hypoglycemia in this patient?