diff --git a/gmid.c b/gmid.c
index 6f349a6..5d6e1cc 100644
--- a/gmid.c
+++ b/gmid.c
@@ -532,6 +532,18 @@ setup_configless(int argc, char **argv, const char *cgi)
 	imsg_flush(&logibuf);
 }
 
+static int
+parse_portno(const char *p)
+{
+	const char *errstr;
+	int n;
+
+	n = strtonum(p, 0, UINT16_MAX, &errstr);
+	if (errstr != NULL)
+		yyerror("port number is %s: %s", errstr, p);
+	return n;
+}
+
 int
 main(int argc, char **argv)
 {
diff --git a/gmid.h b/gmid.h
index 97870df..9a06656 100644
--- a/gmid.h
+++ b/gmid.h
@@ -317,7 +317,6 @@ void		 free_config(void);
 void		 drop_priv(void);
 
 void		 yyerror(const char*, ...);
-int		 parse_portno(const char*);
 void		 parse_conf(const char*);
 int		 cmdline_symset(char *);
 
diff --git a/parse.y b/parse.y
index b656f6b..35a918a 100644
--- a/parse.y
+++ b/parse.y
@@ -78,11 +78,11 @@ char	*symget(const char *);
 
 struct vhost	*new_vhost(void);
 struct location	*new_location(void);
-int		 parse_portno(const char*);
 char		*ensure_absolute_path(char*);
 int		 check_block_code(int);
 char		*check_block_fmt(char*);
 int		 check_strip_no(int);
+int		 check_port_num(int);
 int		 check_prefork_num(int);
 void		 advance_loc(void);
 void		 only_once(const void*, const char*);
@@ -190,7 +190,7 @@ option		: TCHROOT string	{ conf.chroot = $2; }
 			add_mime(&conf.mime, $2, $3);
 		}
 		| TMAP string TTOEXT string { add_mime(&conf.mime, $2, $4); }
-		| TPORT NUM		{ conf.port = $2; }
+		| TPORT NUM		{ conf.port = check_port_num($2); }
 		| TPREFORK NUM		{ conf.prefork = check_prefork_num($2); }
 		| TPROTOCOLS string {
 			if (tls_config_parse_protocols(&conf.protos, $2) == -1)
@@ -864,18 +864,6 @@ new_location(void)
 	return l;
 }
 
-int
-parse_portno(const char *p)
-{
-	const char *errstr;
-	int n;
-
-	n = strtonum(p, 0, UINT16_MAX, &errstr);
-	if (errstr != NULL)
-		yyerror("port number is %s: %s", errstr, p);
-	return n;
-}
-
 char *
 ensure_absolute_path(char *path)
 {
@@ -923,6 +911,16 @@ check_strip_no(int n)
 	return n;
 }
 
+int
+check_port_num(int n)
+{
+	if (n <= 0 || n >= UINT16_MAX)
+		yyerror("port number is %s: %d",
+		    n <= 0 ? "too small" : "too large",
+		    n);
+	return n;
+}
+
 int
 check_prefork_num(int n)
 {