diff --git a/lib/classes/GraphStream.ts b/lib/classes/GraphStream.ts
index 6bcc1ff..33430b3 100644
--- a/lib/classes/GraphStream.ts
+++ b/lib/classes/GraphStream.ts
@@ -2,7 +2,10 @@
 /* eslint-disable no-inline-comments */
 import { Transform } from 'stream';
 import { GraphStreamJSON } from '../interfaces/GraphStreamJSON';
-import { GraphStreamCommand } from '../enums/GraphStreamCommand';
+import {
+  GraphStreamCommand,
+  translate_command
+} from '../enums/GraphStreamCommand';
 import { validate_name } from '../Helper';
 
 interface Stringable {
@@ -12,7 +15,7 @@ interface Stringable {
 
 export class GraphStream extends Transform {
   private _path: string[] = [];
-  private _state = '';
+  private _state: GraphStreamCommand | '' = '';
   private _directional = false;
 
   public get path (): string {
@@ -24,7 +27,7 @@ export class GraphStream extends Transform {
   }
 
   private expect_state (instr: GraphStreamCommand): void {
-    const states = [];
+    const states: (GraphStreamCommand|'')[] = [];
     if ([
       'cug',
       'cdg'
@@ -50,9 +53,11 @@ export class GraphStream extends Transform {
         break;
     }
     if (!states.includes (this._state)) {
-      throw new Error (`invalid state to execute command ${instr}
-  expected: ${states.join (', ')}
-  actual: ${this._state}`);
+      throw new Error (`invalid state to execute command ${
+        translate_command (instr)}
+  expected: ${states.map ((s) => translate_command (s))
+    .join (', ')}
+  actual: ${translate_command (this._state)}`);
     }
   }
 
diff --git a/lib/enums/GraphStreamCommand.ts b/lib/enums/GraphStreamCommand.ts
index d5e7eab..c1fef6a 100644
--- a/lib/enums/GraphStreamCommand.ts
+++ b/lib/enums/GraphStreamCommand.ts
@@ -1,11 +1,28 @@
 /* eslint-disable line-comment-position */
 /* eslint-disable no-inline-comments */
-export type GraphStreamCommand =
-  'cn'| // create node
-  'en'| // end node
-  'cug'| // create unordered graph
-  'cdg'| // create directional graph
-  'csg'| // create subgraph
-  'eg'| // end graph
-  'at'| // add attributes
-  'ce' // create edge
+type GraphStreamCommand =
+  'cn'|
+  'en'|
+  'cug'|
+  'cdg'|
+  'csg'|
+  'eg'|
+  'at'|
+  'ce'
+
+function translate_command (cmd: GraphStreamCommand|''): string {
+  const translations = {
+    cn:  'create node',
+    en:  'end node',
+    cug: 'create unordered graph',
+    cdg: 'create directional graph',
+    csg: 'create subgraph',
+    eg:  'end graph',
+    at:  'attributes',
+    ce:  'create edge',
+    '':  'start'
+  };
+  return translations[cmd];
+}
+
+export { GraphStreamCommand, translate_command };