diff --git a/assembler.js b/assembler.js
index def5161..378d3c0 100644
--- a/assembler.js
+++ b/assembler.js
@@ -212,21 +212,19 @@ function decodeInstructions(source) {
     if (line.type === 'code') {
       const op = line.operation;
 
-      let sourceArgument; // TODO remove null?
-      if (typeof line.argument != 'undefined') {
-        sourceArgument = line.argument;
-      } else {
+      if (typeof line.argument === 'undefined') {
         // If this isn't a label definition,
         // or one of the ops with optional arguments,
         //  then it's an error
         if (!line.operation.startsWith('@')) {
-          if (mnemonicsWithOptionalArgs.indexOf(line.operation) < 0) {
-            console.error(`Missing opcode for line ${line.number}: ${line.source}`);
-            throw new Error("Missing opcode");
+          if (mnemonicsWithOptionalArgs.indexOf(line.operation.toLowerCase()) < 0) {
+            console.error(`Missing operand for line ${line.number}: ${line.source}`);
+            console.error('');
+            throw new Error("Missing operand");
           } else {
             // It *is* one of the special optional-arg ops
             // So let's fill in the implicit operand with $00
-            sourceArgument = '0';
+            line.argument = '0';
           }
         }
       }
@@ -242,13 +240,13 @@ function decodeInstructions(source) {
 
       // Opcodes - Handle constant definitions
       if (op.startsWith(ASM_CONSTANT_PREFIX)) {
-        constants = handleConstantDefinitions(op, sourceArgument, IP, constants);
+        constants = handleConstantDefinitions(op, line.argument, IP, constants);
         continue;
       }
 
       // Opcodes - Handle setting value of IP
       if (op.startsWith(ASM_IP_LABEL)) {
-        IP = parseInt(sourceArgument);
+        IP = parseInt(line.argument);
         continue;
       }
 
@@ -267,7 +265,7 @@ function decodeInstructions(source) {
       line.operation = line.operation.toLowerCase();
 
       // Operands - Handle references to labels
-      if (sourceArgument.startsWith(ASM_LABEL_PREFIX)) {
+      if (line.argument.startsWith(ASM_LABEL_PREFIX)) {
         let label = line.argument.substring(1); // strip label prefix
         if (label in labels) {
           dbg(1, `'${label}' already in labels object`);
@@ -285,30 +283,30 @@ function decodeInstructions(source) {
       }
       
       // Operands - Handle references to the Instruction Pointer
-      if (sourceArgument === ASM_IP_LABEL) {
+      if (line.argument === ASM_IP_LABEL) {
         dbg(1, `  References current IP - ${IP}`);
         decodedArg = IP;
       }
 
       // Operands - Handle references to constants
-      if (sourceArgument.startsWith(ASM_CONSTANT_PREFIX)) {
-        dbg(1, `References '${sourceArgument}'`);
-        decodedArg = decodeNumericOp(constants[sourceArgument.substring(1)]); // substring(1) strips '>'
+      if (line.argument.startsWith(ASM_CONSTANT_PREFIX)) {
+        dbg(1, `References '${line.argument}'`);
+        decodedArg = decodeNumericOp(constants[line.argument.substring(1)]); // substring(1) strips '>'
       }
 
       // Operands - Handle references to constants in indirect mode
-      if (sourceArgument.startsWith(`(${ASM_CONSTANT_PREFIX}`)) {
+      if (line.argument.startsWith(`(${ASM_CONSTANT_PREFIX}`)) {
         addressingMode = "indirect";
-        dbg(1, `(Indirectly) References '${sourceArgument}'`);
-        let constName = sourceArgument.replace(`(${ASM_CONSTANT_PREFIX}`, "");
+        dbg(1, `(Indirectly) References '${line.argument}'`);
+        let constName = line.argument.replace(`(${ASM_CONSTANT_PREFIX}`, "");
         constName = constName.replace(")", "");
         decodedArg = decodeNumericOp(constants[constName]);
       }
 
       // Operands - Handle indirect expressions
-      if (decodedArg === null && sourceArgument.startsWith("(")) {
+      if (decodedArg === null && line.argument.startsWith("(")) {
         addressingMode = "indirect";
-        let indyTemp = sourceArgument.replace("(", "").replace(")", "");
+        let indyTemp = line.argument.replace("(", "").replace(")", "");
         decodedArg = decodeNumericOp(indyTemp);
       }
 
@@ -325,12 +323,12 @@ function decodeInstructions(source) {
       machineCode[IP] = decodedOp;
       machineCode[IP + 1] = decodedArg;
 
-    debugInfo[IP] = {
-      lineNumber: line.number,
-      source: line.source,
-      address: IP,
-      machine: [decodedOp, decodedArg]
-    };
+      debugInfo[IP] = {
+        lineNumber: line.number,
+        source: line.source,
+        address: IP,
+        machine: [decodedOp, decodedArg]
+      };
 
 
       dbg(3, ``);