diff --git a/src/examples/nfc-mfclassic.c b/src/examples/nfc-mfclassic.c
index 732af1a..c164c6e 100644
--- a/src/examples/nfc-mfclassic.c
+++ b/src/examples/nfc-mfclassic.c
@@ -353,12 +353,12 @@ int main(int argc, const char* argv[])
   if(strcmp(command, "r") == 0)
   {
     atAction = ACTION_READ;
-    bUseKeyA = (tolower(*(argv[2])) == 'a');
+    bUseKeyA = tolower((int)((unsigned char)*(argv[2]))) == 'a';
     bUseKeyFile = (argc > 4);
   } else if(strcmp(command, "w") == 0) 
   {
     atAction = ACTION_WRITE;
-    bUseKeyA = (tolower(*(argv[2])) == 'a');
+    bUseKeyA = tolower((int)((unsigned char)*(argv[2]))) == 'a';
     bUseKeyFile = (argc > 4);
   } else if(strcmp(command, "x") == 0)
   {
diff --git a/src/examples/nfc-mfultralight.c b/src/examples/nfc-mfultralight.c
index 1d3e304..7779ff4 100644
--- a/src/examples/nfc-mfultralight.c
+++ b/src/examples/nfc-mfultralight.c
@@ -151,7 +151,7 @@ int main(int argc, const char* argv[])
 
   printf("\nChecking arguments and settings\n");
 
-  bReadAction = (tolower(*(argv[1])) == 'r');
+  bReadAction = tolower((int)((unsigned char)*(argv[1])) == 'r');
 
   if (bReadAction)
   {