diff --git a/pgxn/neon/neon_perf_counters.c b/pgxn/neon/neon_perf_counters.c
index eecbfe98c6..9bce81bf2e 100644
--- a/pgxn/neon/neon_perf_counters.c
+++ b/pgxn/neon/neon_perf_counters.c
@@ -27,7 +27,8 @@ NeonPerfCountersShmemSize(void)
 {
 	Size		size = 0;
 
-	size = add_size(size, mul_size(MaxBackends, sizeof(neon_per_backend_counters)));
+	size = add_size(size, mul_size(NUM_NEON_PERF_COUNTER_SLOTS,
+								   sizeof(neon_per_backend_counters)));
 
 	return size;
 }
@@ -39,7 +40,7 @@ NeonPerfCountersShmemInit(void)
 
 	neon_per_backend_counters_shared =
 		ShmemInitStruct("Neon perf counters",
-						mul_size(MaxBackends,
+						mul_size(NUM_NEON_PERF_COUNTER_SLOTS,
 								 sizeof(neon_per_backend_counters)),
 						&found);
 	Assert(found == IsUnderPostmaster);
@@ -192,7 +193,7 @@ neon_get_backend_perf_counters(PG_FUNCTION_ARGS)
 	/* We put all the tuples into a tuplestore in one go. */
 	InitMaterializedSRF(fcinfo, 0);
 
-	for (int procno = 0; procno < MaxBackends; procno++)
+	for (int procno = 0; procno < NUM_NEON_PERF_COUNTER_SLOTS; procno++)
 	{
 		PGPROC	   *proc = GetPGProcByNumber(procno);
 		int			pid = proc->pid;
@@ -231,7 +232,7 @@ neon_get_perf_counters(PG_FUNCTION_ARGS)
 	InitMaterializedSRF(fcinfo, 0);
 
 	/* Aggregate the counters across all backends */
-	for (int procno = 0; procno < MaxBackends; procno++)
+	for (int procno = 0; procno < NUM_NEON_PERF_COUNTER_SLOTS; procno++)
 	{
 		neon_per_backend_counters *counters = &neon_per_backend_counters_shared[procno];
 
diff --git a/pgxn/neon/neon_perf_counters.h b/pgxn/neon/neon_perf_counters.h
index 02163ada55..49d477c4f8 100644
--- a/pgxn/neon/neon_perf_counters.h
+++ b/pgxn/neon/neon_perf_counters.h
@@ -96,6 +96,14 @@ typedef struct
 /* Pointer to the shared memory array of neon_per_backend_counters structs */
 extern neon_per_backend_counters *neon_per_backend_counters_shared;
 
+/*
+ * Size of the perf counters array in shared memory. One slot for each backend
+ * and aux process. IOW one for each PGPROC slot, except for slots reserved
+ * for prepared transactions, because they're not real processes and cannot do
+ * I/O.
+ */
+#define NUM_NEON_PERF_COUNTER_SLOTS (MaxBackends + NUM_AUXILIARY_PROCS)
+
 #if PG_VERSION_NUM >= 170000
 #define MyNeonCounters (&neon_per_backend_counters_shared[MyProcNumber])
 #else
diff --git a/pgxn/neon/pagestore_smgr.c b/pgxn/neon/pagestore_smgr.c
index 1c87f4405c..155756f8b3 100644
--- a/pgxn/neon/pagestore_smgr.c
+++ b/pgxn/neon/pagestore_smgr.c
@@ -1773,6 +1773,20 @@ neon_init(void)
 	if (MyPState != NULL)
 		return;
 
+	/*
+	 * Sanity check that theperf counters array is sized correctly. We got
+	 * this wrong once, and the formula for max number of backends and aux
+	 * processes might well change in the future, so better safe than sorry.
+	 * This is a very cheap check so we do it even without assertions.  On
+	 * v14, this gets called before initializing MyProc, so we cannot perform
+	 * the check here. That's OK, we don't expect the logic to change in old
+	 * releases.
+	 */
+#if PG_VERSION_NUM>=150000
+	if (MyNeonCounters >= &neon_per_backend_counters_shared[NUM_NEON_PERF_COUNTER_SLOTS])
+		elog(ERROR, "MyNeonCounters points past end of array");
+#endif
+
 	prfs_size = offsetof(PrefetchState, prf_buffer) +
 		sizeof(PrefetchRequest) * readahead_buffer_size;