diff --git a/prism/templates/src/diagnostic.c.erb b/prism/templates/src/diagnostic.c.erb
index a3aa790287..06330c8d5b 100644
--- a/prism/templates/src/diagnostic.c.erb
+++ b/prism/templates/src/diagnostic.c.erb
@@ -99,7 +99,7 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     [PM_ERR_ARGUMENT_FORMAL_IVAR]               = { "invalid formal argument; formal argument cannot be an instance variable", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_ARGUMENT_FORWARDING_UNBOUND]        = { "unexpected `...` in an non-parenthesized call", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_ARGUMENT_IN]                        = { "unexpected `in` keyword in arguments", PM_ERROR_LEVEL_SYNTAX },
-    [PM_ERR_ARGUMENT_NO_FORWARDING_AMP]         = { "unexpected `&` when the parent method is not forwarding", PM_ERROR_LEVEL_SYNTAX },
+    [PM_ERR_ARGUMENT_NO_FORWARDING_AMP]         = { "unexpected `&` no anonymous block parameter", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_ARGUMENT_NO_FORWARDING_ELLIPSES]    = { "unexpected ... when the parent method is not forwarding", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_ARGUMENT_NO_FORWARDING_STAR]        = { "unexpected `*`; no anonymous rest parameter", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_ARGUMENT_NO_FORWARDING_STAR_STAR]   = { "unexpected `**`; no anonymous keyword rest parameter", PM_ERROR_LEVEL_SYNTAX },