diff --git a/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/typecheck/tc-expr-unit.rkt b/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/typecheck/tc-expr-unit.rkt
index a1238399..77a27ae4 100644
--- a/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/typecheck/tc-expr-unit.rkt
+++ b/pkgs/typed-racket-pkgs/typed-racket-lib/typed-racket/typecheck/tc-expr-unit.rkt
@@ -209,7 +209,7 @@
        (match-let* ([(tc-result1: id-t) (single-value #'id)]
                     [(tc-result1: val-t) (single-value #'val)])
          (unless (subtype val-t id-t)
-           (tc-error/expr "Mutation only allowed with compatible types:\n~a is not a subtype of ~a" val-t id-t))
+           (type-mismatch id-t val-t "mutation only allowed with compatible types"))
          (ret -Void))]
       ;; top-level variable reference - occurs at top level
       [(#%top . id) (tc-id #'id)]
@@ -375,7 +375,7 @@
        (match-let* ([(tc-result1: id-t) (tc-expr #'id)]
                     [(tc-result1: val-t) (tc-expr #'val)])
          (unless (subtype val-t id-t)
-           (tc-error/expr "Mutation only allowed with compatible types:\n~a is not a subtype of ~a" val-t id-t))
+           (type-mismatch id-t val-t "mutation only allowed with compatible types"))
          (ret -Void))]
       ;; top-level variable reference - occurs at top level
       [(#%top . id) (tc-id #'id)]