diff --git a/src/tz390.java b/src/tz390.java
index 6e124916..aa321ef4 100644
--- a/src/tz390.java
+++ b/src/tz390.java
@@ -332,6 +332,7 @@ public  class  tz390 {
     * 2024-08-01 AFK #543 Correct OPTABLE(XA,LIST) output to match HLASM
     * 2024-08-12 #545 Extend generated java doco to include private methods
     * 2024-08-15 AFK #554 Correct OPTABLE(ESA,LIST) output to match HLASM
+    * 2024-09-04 #564 fix invalid argument in String compare
 	********************************************************
     * Shared z390 tables                  (last RPI)
     *****************************************************/
@@ -4085,7 +4086,7 @@ else if (override.substring(0,i).equals(String.valueOf(mask_char)))  // #495
                 if (i == -1) // No separator!
                    {abort_error(56,"Missing equal-sign in second override in opcode definition " + op_tables[index]);
                     }
-                else if (override.substring(0,i).equals(mask_char))  // #495
+                else if (override.substring(0,i).equals(String.valueOf(mask_char)))  // #495 #564
                    {override_masks=override.substring(i+1);          // #495
                     }                                                // #495
                 else