diff --git a/ruma-api-macros/src/api/request.rs b/ruma-api-macros/src/api/request.rs
index a4c271a7..5028724d 100644
--- a/ruma-api-macros/src/api/request.rs
+++ b/ruma-api-macros/src/api/request.rs
@@ -144,6 +144,11 @@ impl From<Vec<Field>> for Request {
                     Meta::Word(ident) => {
                         match &ident.to_string()[..] {
                             "body" => {
+                                assert!(
+                                    !has_newtype_body,
+                                    "ruma_api! body attribute can only be used once per request definition"
+                                );
+
                                 has_newtype_body = true;
                                 field_kind = RequestFieldKind::NewtypeBody;
                             }
diff --git a/ruma-api-macros/src/api/response.rs b/ruma-api-macros/src/api/response.rs
index b0ad5165..8bf74528 100644
--- a/ruma-api-macros/src/api/response.rs
+++ b/ruma-api-macros/src/api/response.rs
@@ -116,6 +116,10 @@ impl From<Vec<Field>> for Response {
                             ident == "body",
                             "ruma_api! single-word attribute on responses must be: body"
                         );
+                        assert!(
+                            !has_newtype_body,
+                            "ruma_api! body attribute can only be used once per response definition"
+                        );
 
                         has_newtype_body = true;
                         field_kind = ResponseFieldKind::NewtypeBody;