diff --git a/5.1/assembler.lua b/5.1/assembler.lua
index fb89b18df2..e5f0b2753b 100644
--- a/5.1/assembler.lua
+++ b/5.1/assembler.lua
@@ -100,7 +100,7 @@
   label <- <name> ":"
   string <- '"' ('\\' / '\"' / !'"' .)* '"'
   instruction <- ( <s> ( {<label>} / <number>)? <s> <ln>? <s> {<op>} ( <s> {<param>} )+ <s> ( %nl )+ ) -> {}
-  op <- [A-Z]+
+  op <- [A-Za-z]+
   param <- <register> / <number> / <name> / <string>
   register <- ( "R[" <number> "]" )
   s <- ( !%nl %s )*
@@ -310,10 +310,10 @@
   local b = 3
   local c = 4
   if check_label(table[1]) == nil then
-    op = table[1]
+    op = string.upper (table[1])
   else
     set_label(table[1], parsed, n)
-    op = table[2]
+    op = string.upper (table[2])
     a = 3
     b = 4
     c = 5