diff --git a/kotlin/forth/src/main/kotlin/ForthEvaluator.kt b/kotlin/forth/src/main/kotlin/ForthEvaluator.kt
index 5cdfd8f..6979fd3 100644
--- a/kotlin/forth/src/main/kotlin/ForthEvaluator.kt
+++ b/kotlin/forth/src/main/kotlin/ForthEvaluator.kt
@@ -15,11 +15,19 @@ class ForthEvaluator {
             }
         }
         return emptyList()
+
     }
-    // TODO: This needs to actually work?
     private fun validateToken(token: String): Boolean {
-        require(if (token.toInt() >= 0){ true } else token in validOps)
-        return false
+        if (token !in validOps) {
+            try {
+                token.toInt()
+            } catch (e: NumberFormatException) {
+                if (token !in validOps) {
+                    throw IllegalArgumentException("No definition available for operator \"$token\"")
+                }
+            }
+        }
+        return true
     }
 }