diff --git a/src/apps/netbiosns/netbiosns.c b/src/apps/netbiosns/netbiosns.c
index 4b5cdb87..e1dd3157 100644
--- a/src/apps/netbiosns/netbiosns.c
+++ b/src/apps/netbiosns/netbiosns.c
@@ -499,7 +499,7 @@ netbiosns_set_name(const char *hostname)
 
   /* make name into upper case */
   for (i = 0; i < copy_len; i++ ) {
-    netbiosns_local_name[i] = (char)toupper((unsigned char)hostname[i]);
+    netbiosns_local_name[i] = (char)lwip_toupper(hostname[i]);
   }
   netbiosns_local_name[copy_len] = '\0';
 }
diff --git a/src/include/lwip/arch.h b/src/include/lwip/arch.h
index aa45bee3..ba6204ed 100644
--- a/src/include/lwip/arch.h
+++ b/src/include/lwip/arch.h
@@ -223,6 +223,7 @@ typedef int ssize_t;
 #define lwip_isspace(c)           ((c) == ' ' || (c) == '\f' || (c) == '\n' || (c) == '\r' || (c) == '\t' || (c) == '\v')
 #define lwip_isupper(c)           lwip_in_range((c), 'A', 'Z')
 #define lwip_tolower(c)           (lwip_isupper(c) ? (c) - 'A' + 'a' : c)
+#define lwip_toupper(c)           (lwip_islower(c) ? (c) - 'a' + 'A' : c)
 #else
 #include <ctype.h>
 #define lwip_isdigit(c)           isdigit((unsigned char)(c))
@@ -230,6 +231,7 @@ typedef int ssize_t;
 #define lwip_islower(c)           islower((unsigned char)(c))
 #define lwip_isspace(c)           isspace((unsigned char)(c))
 #define lwip_tolower(c)           tolower((unsigned char)(c))
+#define lwip_toupper(c)           toupper((unsigned char)(c))
 #endif
 
 /** C++ const_cast<target_type>(val) equivalent to remove constness from a value (GCC -Wcast-qual) */