diff --git a/py65/utils/conversions.py b/py65/utils/conversions.py
index 79dfea3..36bfba5 100644
--- a/py65/utils/conversions.py
+++ b/py65/utils/conversions.py
@@ -18,14 +18,14 @@ def itoa(num, base=10):
 
 
 def convert_to_bin(bcd):
-    return bcd2bin[bcd]
+    return _bcd2bin[bcd]
 
 
 def convert_to_bcd(bin):
-    return bin2bcd[bin]
+    return _bin2bcd[bin]
 
 
-bcd2bin = [
+_bcd2bin = (
     0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 10, 11, 12, 13,
     14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 20, 21, 22, 23, 24, 25,
     26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 30, 31, 32, 33, 34, 35, 36, 37,
@@ -42,10 +42,10 @@ bcd2bin = [
     140, 141, 142, 143, 144, 145, 140, 141, 142, 143, 144, 145, 146, 147,
     148, 149, 150, 151, 152, 153, 154, 155, 150, 151, 152, 153, 154, 155,
     156, 157, 158, 159, 160, 161, 162, 163, 164, 165
-]
+)
 
 
-bin2bcd = [
+_bin2bcd = (
     0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09,
     0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19,
     0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29,
@@ -56,4 +56,4 @@ bin2bcd = [
     0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79,
     0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89,
     0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98, 0x99
-]
+)