diff --git a/drivers/isdn/i4l/isdn_common.c b/drivers/isdn/i4l/isdn_common.c
index b26e509ec759..eb21063e6f63 100644
--- a/drivers/isdn/i4l/isdn_common.c
+++ b/drivers/isdn/i4l/isdn_common.c
@@ -340,6 +340,16 @@ isdn_command(isdn_ctrl *cmd)
 		printk(KERN_WARNING "isdn_command command(%x) driver -1\n", cmd->command);
 		return(1);
 	}
+	if (!dev->drv[cmd->driver]) {
+		printk(KERN_WARNING "isdn_command command(%x) dev->drv[%d] NULL\n",
+			cmd->command, cmd->driver);
+		return(1);
+	}
+	if (!dev->drv[cmd->driver]->interface) {
+		printk(KERN_WARNING "isdn_command command(%x) dev->drv[%d]->interface NULL\n",
+			cmd->command, cmd->driver);
+		return(1);
+	}
 	if (cmd->command == ISDN_CMD_SETL2) {
 		int idx = isdn_dc2minor(cmd->driver, cmd->arg & 255);
 		unsigned long l2prot = (cmd->arg >> 8) & 255;
@@ -1903,6 +1913,11 @@ isdn_free_channel(int di, int ch, int usage)
 {
 	int i;
 
+	if ((di < 0) || (ch < 0)) {
+		printk(KERN_WARNING "%s: called with invalid drv(%d) or channel(%d)\n",
+			__FUNCTION__, di, ch);
+		return;
+	}
 	for (i = 0; i < ISDN_MAX_CHANNELS; i++)
 		if (((!usage) || ((dev->usage[i] & ISDN_USAGE_MASK) == usage)) &&
 		    (dev->drvmap[i] == di) &&
@@ -1918,7 +1933,8 @@ isdn_free_channel(int di, int ch, int usage)
 			dev->v110[i] = NULL;
 // 20.10.99 JIM, try to reinitialize v110 !
 			isdn_info_update();
-			skb_queue_purge(&dev->drv[di]->rpqueue[ch]);
+			if (dev->drv[di])
+				skb_queue_purge(&dev->drv[di]->rpqueue[ch]);
 		}
 }