diff --git a/minidlnatypes.h b/minidlnatypes.h
index bda1959..addb754 100644
--- a/minidlnatypes.h
+++ b/minidlnatypes.h
@@ -50,8 +50,8 @@ struct runtime_vars_s {
 
 struct string_s {
 	char *data; // ptr to start of memory area
-	int off;
-	int size;
+	size_t off;
+	size_t size;
 };
 
 typedef uint8_t media_types;
diff --git a/utils.c b/utils.c
index c2cb5eb..57ae5a4 100644
--- a/utils.c
+++ b/utils.c
@@ -37,11 +37,16 @@ inline int
 strcatf(struct string_s *str, const char *fmt, ...)
 {
 	int ret;
+	int size;
 	va_list ap;
 
+	if (str->off >= str->size)
+		return 0;
+
 	va_start(ap, fmt);
-	ret = vsnprintf(str->data + str->off, str->size - str->off, fmt, ap);
-	str->off += ret;
+	size = str->size - str->off;
+	ret = vsnprintf(str->data + str->off, size, fmt, ap);
+	str->off += MIN(ret, size);
 	va_end(ap);
 
 	return ret;