diff --git a/src/bts.cpp b/src/bts.cpp
index cf39aa41..650d4a37 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -338,6 +338,9 @@ struct gprs_rlcmac_bts* bts_alloc(struct gprs_pcu *pcu, uint8_t bts_nr)
 
 void bts_set_current_frame_number(struct gprs_rlcmac_bts *bts, uint32_t fn)
 {
+	/* See also 3GPP TS 45.002, section 4.3.3 */
+	OSMO_ASSERT(fn < GSM_TDMA_HYPERFRAME);
+
 	/* The UL frame numbers lag 3 behind the DL frames and the data
 	 * indication is only sent after all 4 frames of the block have been
 	 * received. Sometimes there is an idle frame between the end of one
diff --git a/tests/fn/FnTest.cpp b/tests/fn/FnTest.cpp
index e5ac46d4..ae15164c 100644
--- a/tests/fn/FnTest.cpp
+++ b/tests/fn/FnTest.cpp
@@ -92,13 +92,13 @@ static void run_test()
 	fn = calc_fn(bts, RFN_MODULUS - 1);
 	OSMO_ASSERT(fn == 42431);
 
-	set_fn(bts, RFN_MODULUS * 123 + 16);
+	set_fn(bts, RFN_MODULUS * 12 + 16);
 	fn = calc_fn(bts, RFN_MODULUS - 4);
-	OSMO_ASSERT(fn == 5219132);
+	OSMO_ASSERT(fn == 509180);
 
-	set_fn(bts, RFN_MODULUS * 123 + 451);
+	set_fn(bts, RFN_MODULUS * 12 + 451);
 	fn = calc_fn(bts, RFN_MODULUS - 175);
-	OSMO_ASSERT(fn == 5218961);
+	OSMO_ASSERT(fn == 509009);
 
 
 	/* Lets check a special cornercase. We assume that
@@ -125,7 +125,7 @@ static void run_test()
 	/* Also check with some corner case
 	 * values where Fn and RFn reach its
 	 * maximum/minimum valid range */
-	set_fn(bts, GSM_MAX_FN);
+	set_fn(bts, GSM_MAX_FN-1);
 	fn = calc_fn(bts, RFN_MODULUS-1);
 	OSMO_ASSERT(fn == GSM_MAX_FN-1);
 
@@ -133,9 +133,9 @@ static void run_test()
 	fn = calc_fn(bts, RFN_MODULUS-1);
 	OSMO_ASSERT(fn == GSM_MAX_FN-1);
 
-	set_fn(bts, GSM_MAX_FN);
+	set_fn(bts, GSM_MAX_FN-1);
 	fn = calc_fn(bts, 0);
-	OSMO_ASSERT(fn == GSM_MAX_FN);
+	OSMO_ASSERT(fn == GSM_MAX_FN-RFN_MODULUS*2);
 
 	set_fn(bts, 0);
 	fn = calc_fn(bts, 0);
diff --git a/tests/fn/FnTest.ok b/tests/fn/FnTest.ok
index be6400fe..4884ca5b 100644
--- a/tests/fn/FnTest.ok
+++ b/tests/fn/FnTest.ok
@@ -19,11 +19,11 @@ rfn=42422 ==> fn=42422
 bts: fn=42433
 rfn=42431 ==> fn=42431
 
-bts: fn=5219152
-rfn=42428 ==> fn=5219132
+bts: fn=509200
+rfn=42428 ==> fn=509180
 
-bts: fn=5219587
-rfn=42257 ==> fn=5218961
+bts: fn=509635
+rfn=42257 ==> fn=509009
 
 bts: fn=0
 rfn=42419 ==> fn=2715635
@@ -37,14 +37,14 @@ rfn=42422 ==> fn=2715638
 bts: fn=23
 rfn=42390 ==> fn=2715606
 
-bts: fn=2715648
+bts: fn=2715647
 rfn=42431 ==> fn=2715647
 
 bts: fn=0
 rfn=42431 ==> fn=2715647
 
-bts: fn=2715648
-rfn=0 ==> fn=2715648
+bts: fn=2715647
+rfn=0 ==> fn=2630784
 
 bts: fn=0
 rfn=0 ==> fn=0