diff --git a/Python/ceval.c b/Python/ceval.c
index fa14a1c04f1e0c..b69d5aa9d32069 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -4946,6 +4946,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, InterpreterFrame *frame, int thr
             PyObject *inst = TOP();
             int retval = PyObject_IsInstance(inst, cls);
             if (retval < 0) {
+                Py_DECREF(cls);
                 goto error;
             }
             PyObject *res = PyBool_FromLong(retval);