diff --git a/src/compiler/emitter.ts b/src/compiler/emitter.ts
index 1c5a04f60ed..f8ceaf4610f 100644
--- a/src/compiler/emitter.ts
+++ b/src/compiler/emitter.ts
@@ -486,7 +486,7 @@ namespace ts {
                     return pipelineEmitWithHint;
 
                 default:
-                    return Debug.assertNever(phase);
+                    return Debug.assertNever(phase, `Unexpected value for PipelinePhase: ${phase}`);
             }
         }