diff --git a/lib/hyperscript/parser.sx b/lib/hyperscript/parser.sx
index 25aea140..15e05503 100644
--- a/lib/hyperscript/parser.sx
+++ b/lib/hyperscript/parser.sx
@@ -2871,7 +2871,28 @@
                       (list (quote view-transition!) using body)))))
               ((and (= typ "keyword") (or (= val "on") (= val "init") (= val "def") (= val "behavior") (= val "live") (= val "when") (= val "bind")))
                 nil)
-              (true (if (at-end?) nil (parse-expr)))))))
+              (true
+                (if
+                  (at-end?)
+                  nil
+                  (let
+                    ((expr (parse-expr)))
+                    (if
+                      (if
+                        (and (dict? expr) (get expr :hs-ast))
+                        (= (get expr :kind) "arith")
+                        (and
+                          (list? expr)
+                          (let
+                            ((h (first expr)))
+                            (or
+                              (= h (quote +))
+                              (= h (quote -))
+                              (= h (quote *))
+                              (= h (quote /))
+                              (= h (make-symbol "%"))))))
+                      (error "Pseudo-commands must be function calls")
+                      expr))))))))
       (define
         parse-cmd-list
         (fn
diff --git a/shared/static/wasm/sx/hs-parser.sx b/shared/static/wasm/sx/hs-parser.sx
index 25aea140..15e05503 100644
--- a/shared/static/wasm/sx/hs-parser.sx
+++ b/shared/static/wasm/sx/hs-parser.sx
@@ -2871,7 +2871,28 @@
                       (list (quote view-transition!) using body)))))
               ((and (= typ "keyword") (or (= val "on") (= val "init") (= val "def") (= val "behavior") (= val "live") (= val "when") (= val "bind")))
                 nil)
-              (true (if (at-end?) nil (parse-expr)))))))
+              (true
+                (if
+                  (at-end?)
+                  nil
+                  (let
+                    ((expr (parse-expr)))
+                    (if
+                      (if
+                        (and (dict? expr) (get expr :hs-ast))
+                        (= (get expr :kind) "arith")
+                        (and
+                          (list? expr)
+                          (let
+                            ((h (first expr)))
+                            (or
+                              (= h (quote +))
+                              (= h (quote -))
+                              (= h (quote *))
+                              (= h (quote /))
+                              (= h (make-symbol "%"))))))
+                      (error "Pseudo-commands must be function calls")
+                      expr))))))))
       (define
         parse-cmd-list
         (fn