diff --git a/src/esp32/esp32_spi_master.c b/src/esp32/esp32_spi_master.c
index 5cd1870..e19f2f8 100644
--- a/src/esp32/esp32_spi_master.c
+++ b/src/esp32/esp32_spi_master.c
@@ -264,7 +264,8 @@ static void esp32_spi_get_rx_data(spi_dev_t *dev, uint8_t *data, size_t skip,
       if (skip > 0) {
         skip--;
       } else {
-        data[i++] = byte;
+        if (data != NULL) data[i] = byte;
+        ++i;
       }
       w >>= 8;
     }
@@ -314,7 +315,7 @@ static bool mgos_spi_run_txn_fd(struct mgos_spi *c, const void *tx_data,
     }
     esp32_spi_get_rx_data(dev, rxdp, 0, dlen);
     txdp += dlen;
-    rxdp += dlen;
+    if (rxdp != NULL) rxdp += dlen;
   }
 
   return true;
diff --git a/src/stm32/stm32_spi_master_gspi.c b/src/stm32/stm32_spi_master_gspi.c
index de077f8..e7d43bf 100644
--- a/src/stm32/stm32_spi_master_gspi.c
+++ b/src/stm32/stm32_spi_master_gspi.c
@@ -102,6 +102,7 @@ bool stm32_gspi_run_txn_fd(struct mgos_spi *c, const struct mgos_spi_txn *txn) {
   size_t len = txn->fd.len;
   const uint8_t *tx_data = (const uint8_t *) txn->fd.tx_data;
   uint8_t *rx_data = (uint8_t *) txn->fd.rx_data;
+  bool discard_rx = (txn->fd.rx_data == NULL);
   volatile uint8_t *drp = (volatile uint8_t *) &c->regs->DR;
 
   if (c->debug) {
@@ -128,7 +129,7 @@ bool stm32_gspi_run_txn_fd(struct mgos_spi *c, const struct mgos_spi_txn *txn) {
       if (c->debug) {
         LOG(LL_DEBUG, ("read 0x%02x", byte));
       }
-      *rx_data++ = byte;
+      if (!discard_rx) *rx_data++ = byte;
     }
     len--;
   }