diff --git a/user_io.c b/user_io.c
index 12421c9..0c19c60 100644
--- a/user_io.c
+++ b/user_io.c
@@ -2655,14 +2655,19 @@ char user_io_key_remap(char *s, char action, int tag) {
 
 unsigned char user_io_ext_idx(const char *name, const char* ext) {
 	unsigned char idx = 0;
+	char ext3[4]; // extension truncated or extended to 3 chars
 	int len = strlen(ext);
 	int extlen;
 
 	const char *nameext = GetExtension(name);
 	if (!nameext) return 0;
 	extlen = strlen(nameext);
+	for (int i=0; i<3; i++) {
+		ext3[i] = i<extlen ? nameext[i] : ' ';
+	}
+	ext3[3] = 0;
 	while((len>3) && *ext) {
-		if(!_strnicmp(nameext,ext,extlen > 3 ? 3 : extlen)) return idx;
+		if(!_strnicmp(ext3,ext,3)) return idx;
 		if(strlen(ext)<=3) break;
 		idx++;
 		ext +=3;