diff --git a/src/gprs_rlcmac_ts_alloc.cpp b/src/gprs_rlcmac_ts_alloc.cpp
index e394a6e..c45aa39 100644
--- a/src/gprs_rlcmac_ts_alloc.cpp
+++ b/src/gprs_rlcmac_ts_alloc.cpp
@@ -329,7 +329,7 @@
 /*! Return free TFI
  *
  *  \param[in] bts Pointer to BTS struct
- *  \param[in] trx Pointer to TRX struct
+ *  \param[in] trx Optional pointer to TRX struct
  *  \param[in] ms Pointer to MS object
  *  \param[in] dir DL or UL direction
  *  \param[in] use_trx which TRX to use or -1 if it should be selected based on what MS uses
@@ -342,6 +342,15 @@
 	int tfi;
 	uint8_t trx_no;
 
+	if (trx) {
+		if (use_trx >= 0 && use_trx != trx->trx_no) {
+			LOGP(DRLCMAC, LOGL_ERROR, "- Requested incompatible TRX %d (current is %d)\n",
+			     use_trx, trx->trx_no);
+			return -EINVAL;
+		}
+		use_trx = trx->trx_no;
+	}
+
 	if (use_trx == -1 && ms->current_trx())
 		use_trx = ms->current_trx()->trx_no;
 
@@ -786,16 +795,6 @@
 	first_common_ts = ms->first_common_ts();
 	trx = ms->current_trx();
 
-	if (trx) {
-		if (use_trx >= 0 && use_trx != trx->trx_no) {
-			LOGP(DRLCMAC, LOGL_ERROR,
-				"- Requested incompatible TRX %d (current is %d)\n",
-				use_trx, trx->trx_no);
-			return -EINVAL;
-		}
-		use_trx = trx->trx_no;
-	}
-
 	/* Step 2a: Find usable TRX and TFI */
 	tfi = tfi_find_free(bts->bts, trx, ms, tbf->direction, use_trx, &trx_no);
 	if (tfi < 0) {