diff --git a/pwdx.c b/pwdx.c
index 214275a9..9d409027 100644
--- a/pwdx.c
+++ b/pwdx.c
@@ -52,6 +52,7 @@ int check_pid_argument(char *input)
 
 	if (!strncmp("/proc/", input, 6))
 		skip = 6;
+	errno = 0;
 	pid = strtol(input + skip, &end, 10);
 
 	if (errno || input + skip == end || (end && *end))