diff --git a/sketches/assembler.js b/sketches/assembler.js
index e9cee51..9452a3d 100644
--- a/sketches/assembler.js
+++ b/sketches/assembler.js
@@ -22,27 +22,39 @@ function decodeMultipleInstructions(str) {
   let lines = str.split(/\n/); // returns an array of lines
   let output = [];
   lines.forEach( (l) => {
-    let stripped = stripWhitespaceFromEnds(stripComments(l));
-    let decoded = decodeInstruction(stripped);
-    output.push(decoded.op);
-    output.push(decoded.arg);
+    let decoded = decodeInstruction(l);
+    if (decoded) {
+      output.push(decoded.op);
+      output.push(decoded.arg);
+    }
   });
   return new Uint8Array(output);
 }
 
+/**
+ * @param {string} line - A line of assembly code
+ * @returns {(object|false)} Either {op: machineOp, arg: 0}, or false if the line was blank
+ */
 function decodeInstruction(line) {
-  let addressingMode = "direct"; // Must be "direct" or "indirect"
-  op_arg_array = line.split(" ");
+  line = stripWhitespaceFromEnds(stripComments(line));
+  let op_arg_array = line.split(" "); // split line into an array of [op, arg]
   let opName = op_arg_array[0].toLowerCase();
+  let addressingMode = 'direct'; // Must be "direct" or "indirect"
 
-  // Handle mnemonics without arguments (eg END)
+  // Handle blank lines and mnemonics without arguments (eg END)
   if (op_arg_array.length < 2) { // No argument
+    // handle blank lines, or lines that just contain a comment:
+    if (line.length === 0) { return false; }
+
+    // handle mnemonics that aren't paired with an argument:
     if (mnemonicsWithOptionalArgs.indexOf(opName) < 0) {
-      throw error("Missing opcode");
+      console.error(`Missing opcode: ${line}`);
+      throw new Error("Missing opcode");
     }
     let machineOp = mnemonics2opcodes[opName][addressingMode];
     return { op: machineOp, arg: 0 };
   }
+
   // Handle mnemonics with arguments (eg ADD $FF)
   let arg_str = op_arg_array[1];
   if (arg_str.startsWith("(")) {