diff --git a/drivers/atmodem/sim.c b/drivers/atmodem/sim.c
index 5513932d..c51b1d49 100644
--- a/drivers/atmodem/sim.c
+++ b/drivers/atmodem/sim.c
@@ -305,7 +305,7 @@ static void at_sim_update_file(struct ofono_sim *sim, int cmd, int fileid,
 	struct sim_data *sd = ofono_sim_get_data(sim);
 	struct cb_data *cbd = cb_data_new(cb, data);
 	char *buf;
-	char *quote = "";
+	gboolean quote = FALSE;
 	int len, ret;
 	int size = 36 + p3 * 2;
 
@@ -317,7 +317,7 @@ static void at_sim_update_file(struct ofono_sim *sim, int cmd, int fileid,
 	case OFONO_VENDOR_ZTE:
 	case OFONO_VENDOR_HUAWEI:
 	case OFONO_VENDOR_SPEEDUP:
-		quote = "\"";
+		quote = TRUE;
 		size += 2;
 		break;
 	}
@@ -326,13 +326,16 @@ static void at_sim_update_file(struct ofono_sim *sim, int cmd, int fileid,
 	if (buf == NULL)
 		goto error;
 
-	len = sprintf(buf, "AT+CRSM=%i,%i,%i,%i,%i,%s", cmd, fileid,
-			p1, p2, p3, quote);
+	len = sprintf(buf, "AT+CRSM=%i,%i,%i,%i,%i,", cmd, fileid,p1, p2, p3);
+
+	if (quote)
+		buf[len++] = '\"';
 
 	for (; p3; p3--)
 		len += sprintf(buf + len, "%02hhX", *value++);
 
-	sprintf(buf + len, "%s", quote);
+	if (quote)
+		buf[len++] = '\"';
 
 	ret = g_at_chat_send(sd->chat, buf, crsm_prefix,
 				at_crsm_update_cb, cbd, g_free);