diff --git a/config.yml b/config.yml
index dbf0c9e1e1..d8a10bc113 100644
--- a/config.yml
+++ b/config.yml
@@ -64,6 +64,7 @@ errors:
   - DEF_ENDLESS
   - DEF_ENDLESS_PARAMETERS
   - DEF_ENDLESS_SETTER
+  - DEF_ENDLESS_DO_BLOCK
   - DEF_NAME
   - DEF_PARAMS_TERM
   - DEF_PARAMS_TERM_PAREN
diff --git a/src/prism.c b/src/prism.c
index 2603bf7adb..c644c94753 100644
--- a/src/prism.c
+++ b/src/prism.c
@@ -18998,6 +18998,20 @@ parse_expression_prefix(pm_parser_t *parser, pm_binding_power_t binding_power, b
 
                 pm_node_t *statement = parse_expression(parser, PM_BINDING_POWER_DEFINED + 1, allow_command_call, false, PM_ERR_DEF_ENDLESS, (uint16_t) (depth + 1));
 
+                // In an endless method definition, the body is not allowed to
+                // be a command with a do..end block.
+                if (PM_NODE_TYPE_P(statement, PM_CALL_NODE)) {
+                    pm_call_node_t *call = (pm_call_node_t *) statement;
+
+                    if (call->arguments != NULL && call->block != NULL && PM_NODE_TYPE_P(call->block, PM_BLOCK_NODE)) {
+                        pm_block_node_t *block = (pm_block_node_t *) call->block;
+
+                        if (parser->start[block->opening_loc.start] != '{') {
+                            pm_parser_err_node(parser, call->block, PM_ERR_DEF_ENDLESS_DO_BLOCK);
+                        }
+                    }
+                }
+
                 if (accept1(parser, PM_TOKEN_KEYWORD_RESCUE_MODIFIER)) {
                     context_push(parser, PM_CONTEXT_RESCUE_MODIFIER);
 
diff --git a/templates/src/diagnostic.c.erb b/templates/src/diagnostic.c.erb
index c943c3afb9..d717dc1e16 100644
--- a/templates/src/diagnostic.c.erb
+++ b/templates/src/diagnostic.c.erb
@@ -148,6 +148,7 @@ static const pm_diagnostic_data_t diagnostic_messages[PM_DIAGNOSTIC_ID_MAX] = {
     [PM_ERR_DEF_ENDLESS]                        = { "could not parse the endless method body", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_DEF_ENDLESS_PARAMETERS]             = { "could not parse the endless method parameters", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_DEF_ENDLESS_SETTER]                 = { "invalid method name; a setter method cannot be defined in an endless method definition", PM_ERROR_LEVEL_SYNTAX },
+    [PM_ERR_DEF_ENDLESS_DO_BLOCK]               = { "unexpected `do` for block in an endless method definition", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_DEF_NAME]                           = { "unexpected %s; expected a method name", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_DEF_PARAMS_TERM]                    = { "expected a delimiter to close the parameters", PM_ERROR_LEVEL_SYNTAX },
     [PM_ERR_DEF_PARAMS_TERM_PAREN]              = { "unexpected %s; expected a `)` to close the parameters", PM_ERROR_LEVEL_SYNTAX },
diff --git a/test/prism/errors/def_endless_do.txt b/test/prism/errors/def_endless_do.txt
new file mode 100644
index 0000000000..4d786638a6
--- /dev/null
+++ b/test/prism/errors/def_endless_do.txt
@@ -0,0 +1,3 @@
+def a = a b do 1 end
+            ^~~~~~~~ unexpected `do` for block in an endless method definition
+