diff --git a/templates/src/diagnostic.c.erb b/templates/src/diagnostic.c.erb
index 81123a8e6cd..15b7513154a 100644
--- a/templates/src/diagnostic.c.erb
+++ b/templates/src/diagnostic.c.erb
@@ -128,7 +128,7 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     [PM_ERR_CASE_MISSING_CONDITIONS]            = { "expected a `when` or `in` clause after `case`", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_CASE_TERM]                          = { "expected an `end` to close the `case` statement", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_CLASS_IN_METHOD]                    = { "unexpected class definition in method body", PM_ERROR_LEVEL_SYNTAX },
-    [PM_ERR_CLASS_NAME]                         = { "expected a constant name after `class`", PM_ERROR_LEVEL_SYNTAX },
+    [PM_ERR_CLASS_NAME]                         = { "unexpected constant path after `class`; class/module name must be CONSTANT", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_CLASS_SUPERCLASS]                   = { "expected a superclass after `<`", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_CLASS_TERM]                         = { "expected an `end` to close the `class` statement", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_CLASS_UNEXPECTED_END]               = { "unexpected `end`, expecting ';' or '\\n'", PM_ERROR_LEVEL_SYNTAX },
@@ -233,7 +233,7 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     [PM_ERR_MALLOC_FAILED]                      = { "failed to allocate memory", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_MIXED_ENCODING]                     = { "UTF-8 mixed within %s source", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_MODULE_IN_METHOD]                   = { "unexpected module definition in method body", PM_ERROR_LEVEL_SYNTAX },
-    [PM_ERR_MODULE_NAME]                        = { "expected a constant name after `module`", PM_ERROR_LEVEL_SYNTAX },
+    [PM_ERR_MODULE_NAME]                        = { "unexpected constant path after `module`; class/module name must be CONSTANT", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_MODULE_TERM]                        = { "expected an `end` to close the `module` statement", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_MULTI_ASSIGN_MULTI_SPLATS]          = { "multiple splats in multiple assignment", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_MULTI_ASSIGN_UNEXPECTED_REST]       = { "unexpected '%.*s' resulting in multiple splats in multiple assignment", PM_ERROR_LEVEL_SYNTAX },