diff --git a/assembler.js b/assembler.js
index 44b52b1..ca998d2 100644
--- a/assembler.js
+++ b/assembler.js
@@ -303,6 +303,13 @@ function decodeInstructions(source) {
       // Operands - Handle references to constants
       if (line.argument.startsWith(ASM_CONSTANT_PREFIX)) {
         dbg(1, `References '${line.argument}'`);
+        if (typeof constants[line.argument.substring(1)] === 'undefined') {
+          console.error();
+          console.error(`Error: Undefined constant '${line.argument}'`);
+          console.error(`  at line ${line.number}`);
+          console.error();
+          throw new Error('Undefined constant');
+        }
         decodedArg = decodeNumericOp(constants[line.argument.substring(1)]); // substring(1) strips '>'
       }