diff --git a/src/core/dns.c b/src/core/dns.c
index 05870db0..997a548e 100644
--- a/src/core/dns.c
+++ b/src/core/dns.c
@@ -116,6 +116,13 @@ static u16_t dns_txid;
 #error DNS_MAX_TTL must be a positive 32-bit value
 #endif
 
+#if DNS_TABLE_SIZE > 255
+#error DNS_TABLE_SIZE must fit into an u8_t
+#endif
+#if DNS_MAX_SERVERS > 255
+#error DNS_MAX_SERVERS must fit into an u8_t
+#endif
+
 /* The number of parallel requests (i.e. calls to dns_gethostbyname
  * that cannot be answered from the DNS table.
  * This is set to the table size by default.
@@ -123,6 +130,10 @@ static u16_t dns_txid;
 #if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_NO_MULTIPLE_OUTSTANDING) != 0)
 #ifndef DNS_MAX_REQUESTS
 #define DNS_MAX_REQUESTS          DNS_TABLE_SIZE
+#else
+#if DNS_MAX_REQUESTS > 255
+#error DNS_MAX_REQUESTS must fit into an u8_t
+#endif
 #endif
 #else
 /* In this configuration, both arrays have to have the same size and are used
@@ -134,6 +145,10 @@ static u16_t dns_txid;
 #if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) != 0)
 #ifndef DNS_MAX_SOURCE_PORTS
 #define DNS_MAX_SOURCE_PORTS      DNS_MAX_REQUESTS
+#else
+#if DNS_MAX_SOURCE_PORTS > 255
+#error DNS_MAX_SOURCE_PORTS must fit into an u8_t
+#endif
 #endif
 #else
 #ifdef DNS_MAX_SOURCE_PORTS