diff --git a/nanomodbus.c b/nanomodbus.c
index 47ca67d..1eaebcb 100644
--- a/nanomodbus.c
+++ b/nanomodbus.c
@@ -648,7 +648,7 @@ nmbs_error recv_read_file_record_res(nmbs_t* nmbs, uint16_t* registers, uint16_t
         return err;
 
     uint8_t response_size = get_1(nmbs);
-    if (response_size > 245) {
+    if (response_size > 250) {
         return NMBS_ERROR_INVALID_RESPONSE;
     }
 
@@ -1798,6 +1798,7 @@ nmbs_error nmbs_read_file_record(nmbs_t* nmbs, uint16_t file_number, uint16_t re
     if (record_number > 0x270F)
         return NMBS_ERROR_INVALID_ARGUMENT;
 
+    // In expected response: max PDU length = 253, assuming a single file request, (253 - 1 - 1 - 1 - 1) / 2 = 124
     if (count > 124)
         return NMBS_ERROR_INVALID_ARGUMENT;
 
diff --git a/tests/nanomodbus_tests.c b/tests/nanomodbus_tests.c
index 64ab716..303e740 100644
--- a/tests/nanomodbus_tests.c
+++ b/tests/nanomodbus_tests.c
@@ -934,7 +934,7 @@ void test_fc20(nmbs_transport transport) {
     expect(registers[2] == 0xAA55);
     expect(registers[3] == 0xFFFF);
 
-    check(nmbs_read_file_record(&CLIENT, 255, 9999, registers, 120));
+    check(nmbs_read_file_record(&CLIENT, 255, 9999, registers, 124));
     expect(registers[123] == 42);
 
     stop_client_and_server();