diff --git a/src/writer/spirv/builder.cc b/src/writer/spirv/builder.cc
index 66136a2..47e8a36 100644
--- a/src/writer/spirv/builder.cc
+++ b/src/writer/spirv/builder.cc
@@ -207,7 +207,7 @@
     return GenerateIdentifierExpression(expr->AsIdentifier());
   }
 
-  error_ = "unknown expression type";
+  error_ = "unknown expression type: " + expr->str();
   return 0;
 }