--- a/tcpdump.c
+++ b/tcpdump.c
@@ -66,6 +66,7 @@ extern int SIZE_BUF;
 #include <stdlib.h>
 #include <string.h>
 #include <limits.h>
+#include <ctype.h>
 #ifndef WIN32
 #include <sys/wait.h>
 #include <sys/resource.h>
@@ -625,6 +626,7 @@ main(int argc, char **argv)
 #ifdef HAVE_PCAP_FINDALLDEVS
 	pcap_if_t *devpointer;
 	int devnum;
+	char *devp;
 #endif
 	int status;
 #ifdef WIN32
@@ -775,7 +777,8 @@ main(int argc, char **argv)
 			 * It can be useful on Windows, where more than
 			 * one interface can have the same name.
 			 */
-			if ((devnum = atoi(optarg)) != 0) {
+			for (devp = optarg; *devp && isdigit(*devp); devp++);
+			if ((!*devp || isspace(*devp)) && (devnum = atoi(optarg)) != 0) {
 				if (devnum < 0)
 					error("Invalid adapter index");
 
