diff --git a/libavcodec/xpmdec.c b/libavcodec/xpmdec.c
index b9b2551ec1..605a09d05a 100644
--- a/libavcodec/xpmdec.c
+++ b/libavcodec/xpmdec.c
@@ -188,9 +188,9 @@ static const ColorEntry color_table[] = {
 static unsigned hex_char_to_number(uint8_t x)
 {
     if (x >= 'a' && x <= 'f')
-        x -= 87;
+        x -= 'a' - 10;
     else if (x >= 'A' && x <= 'F')
-        x -= 55;
+        x -= 'A' - 10;
     else if (x >= '0' && x <= '9')
         x -= '0';
     else