diff --git a/openair1/PHY/INIT/nr_init.c b/openair1/PHY/INIT/nr_init.c
index 1f918086b2..864420f56e 100644
--- a/openair1/PHY/INIT/nr_init.c
+++ b/openair1/PHY/INIT/nr_init.c
@@ -135,6 +135,8 @@ int phy_init_nr_gNB(PHY_VARS_gNB *gNB)
 
   load_LDPClib(NULL, &ldpc_interface);
 
+  pthread_mutex_init(&gNB->UL_INFO.crc_rx_mutex, NULL);
+
   if (gNB->ldpc_offload_flag)
     load_LDPClib("_t2", &ldpc_interface_offload);
   gNB->max_nb_pdsch = MAX_MOBILES_PER_GNB;
@@ -331,6 +333,8 @@ void phy_free_nr_gNB(PHY_VARS_gNB *gNB)
   const int max_ul_mimo_layers = 4; // taken from phy_init_nr_gNB()
   const int n_buf = Prx * max_ul_mimo_layers;
 
+  pthread_mutex_destroy(&gNB->UL_INFO.crc_rx_mutex);
+
   PHY_MEASUREMENTS_gNB *meas = &gNB->measurements;
   free_and_zero(meas->n0_subband_power);
   free_and_zero(meas->n0_subband_power_dB);
diff --git a/openair1/SCHED_NR/phy_procedures_nr_gNB.c b/openair1/SCHED_NR/phy_procedures_nr_gNB.c
index 4005c7d581..4289c98a57 100644
--- a/openair1/SCHED_NR/phy_procedures_nr_gNB.c
+++ b/openair1/SCHED_NR/phy_procedures_nr_gNB.c
@@ -471,6 +471,14 @@ void nr_fill_indication(PHY_VARS_gNB *gNB, int frame, int slot_rx, int ULSCH_id,
   else if (SNRtimes10 >  635) cqi=255;
   else                        cqi=(640+SNRtimes10)/5;
 
+  // multiple threads might call this function at the same time, or while the
+  // L2 reads the messages. Hence, if not protected, crc and rx indications
+  // might not appear pairwise (in the same order) in the same slot, or even in
+  // separate slots. The L2 does not support this; hence, use the crc_rx_mutex
+  // to ensure that messages are pairwise.
+  int rc = pthread_mutex_lock(&gNB->UL_INFO.crc_rx_mutex);
+  DevAssert(rc == 0);
+
   // crc indication
   uint16_t num_crc = gNB->UL_INFO.crc_ind.number_crcs;
   gNB->UL_INFO.crc_ind.crc_list = &gNB->crc_pdu_list[0];
@@ -507,8 +515,10 @@ void nr_fill_indication(PHY_VARS_gNB *gNB, int frame, int slot_rx, int ULSCH_id,
     gNB->rx_pdu_list[num_rx].pdu_length = harq_process->TBS;
     gNB->rx_pdu_list[num_rx].pdu = harq_process->b;
   }
-
   gNB->UL_INFO.rx_ind.number_of_pdus++;
+
+  rc = pthread_mutex_unlock(&gNB->UL_INFO.crc_rx_mutex);
+  DevAssert(rc == 0);
 }
 
 // Function to fill UL RB mask to be used for N0 measurements
diff --git a/openair2/NR_PHY_INTERFACE/NR_IF_Module.c b/openair2/NR_PHY_INTERFACE/NR_IF_Module.c
index 37bedddf46..8494a56492 100644
--- a/openair2/NR_PHY_INTERFACE/NR_IF_Module.c
+++ b/openair2/NR_PHY_INTERFACE/NR_IF_Module.c
@@ -177,6 +177,9 @@ void handle_nr_ulsch(NR_UL_IND_t *UL_info)
   }
 
   if (UL_info->rx_ind.number_of_pdus > 0 && UL_info->crc_ind.number_crcs > 0) {
+    // see nr_fill_indication about why this mutex is necessary
+    int rc = pthread_mutex_lock(&UL_info->crc_rx_mutex);
+    DevAssert(rc == 0);
     AssertFatal(UL_info->rx_ind.number_of_pdus == UL_info->crc_ind.number_crcs,
                 "number_of_pdus %d, number_crcs %d\n",
                 UL_info->rx_ind.number_of_pdus, UL_info->crc_ind.number_crcs);
@@ -210,9 +213,11 @@ void handle_nr_ulsch(NR_UL_IND_t *UL_info)
                 rx->rssi);
       handle_nr_ul_harq(UL_info->CC_id, UL_info->module_id, UL_info->frame, UL_info->slot, crc);
     }
+    UL_info->rx_ind.number_of_pdus = 0;
+    UL_info->crc_ind.number_crcs = 0;
+    rc = pthread_mutex_unlock(&UL_info->crc_rx_mutex);
+    DevAssert(rc == 0);
   }
-  UL_info->rx_ind.number_of_pdus = 0;
-  UL_info->crc_ind.number_crcs = 0;
 }
 
 void handle_nr_srs(NR_UL_IND_t *UL_info) {
diff --git a/openair2/NR_PHY_INTERFACE/NR_IF_Module.h b/openair2/NR_PHY_INTERFACE/NR_IF_Module.h
index 108e0afe7a..ba230e4f63 100644
--- a/openair2/NR_PHY_INTERFACE/NR_IF_Module.h
+++ b/openair2/NR_PHY_INTERFACE/NR_IF_Module.h
@@ -64,6 +64,11 @@ typedef struct {
 
   /// crc indication list
   nfapi_nr_crc_indication_t crc_ind;
+  /// RX indication
+  nfapi_nr_rx_data_indication_t rx_ind;
+  /// mutex to protect concurrent access to crc_ind and rx_ind, which the L2
+  /// needs to be pairwise
+  pthread_mutex_t crc_rx_mutex;
 
   /// RACH indication list
   nfapi_nr_rach_indication_t rach_ind;
@@ -71,9 +76,6 @@ typedef struct {
   /// SRS indication list
   nfapi_nr_srs_indication_t srs_ind;
 
-  /// RX indication
-  nfapi_nr_rx_data_indication_t rx_ind;
-
   /// UCI indication
   nfapi_nr_uci_indication_t uci_ind;