diff --git a/src/stdlib/strto_int.c b/src/stdlib/strto_int.c
index 2da6dc7..7c9da4f 100644
--- a/src/stdlib/strto_int.c
+++ b/src/stdlib/strto_int.c
@@ -49,7 +49,7 @@ int __strto_int(struct __scanf_input *input, int base, long *outl,
 		int v = -1;
 		int c = __scanf_peek(input);
 		if(isdigit(c)) v = c - '0';
-		if(islower(c)) v = c - 'a' + 10;
+		if(isalpha(c)) v = tolower(c) - 'a' + 10;
 		if(v == -1 || v >= base) break;
 
 		/* The value is valid as long as there is at least one digit */