diff --git a/flang/lib/Semantics/rewrite-parse-tree.cpp b/flang/lib/Semantics/rewrite-parse-tree.cpp
--- a/flang/lib/Semantics/rewrite-parse-tree.cpp
+++ b/flang/lib/Semantics/rewrite-parse-tree.cpp
@@ -110,6 +110,8 @@
   return true;
 }
 
+// Convert a syntactically ambiguous io-unit internal-file-variable to a
+// file-unit-number.
 void RewriteMutator::Post(parser::IoUnit &x) {
   if (auto *var{std::get_if<parser::Variable>(&x.u)}) {
     const parser::Name &last{parser::GetLastName(*var)};
@@ -118,11 +120,13 @@
       // If the Variable is not known to be character (any kind), transform
       // the I/O unit in situ to a FileUnitNumber so that automatic expression
       // constraint checking will be applied.
+      auto source{var->GetSource()};
       auto expr{std::visit(
           [](auto &&indirection) {
             return parser::Expr{std::move(indirection)};
           },
           std::move(var->u))};
+      expr.source = source;
       x.u = parser::FileUnitNumber{
           parser::ScalarIntExpr{parser::IntExpr{std::move(expr)}}};
     }
diff --git a/flang/test/Semantics/io04.f90 b/flang/test/Semantics/io04.f90
--- a/flang/test/Semantics/io04.f90
+++ b/flang/test/Semantics/io04.f90
@@ -87,6 +87,9 @@
   !ERROR: If UNIT=* appears, REC must not appear
   write(*, rec=13) 'Ok'
 
+  !ERROR: Must have INTEGER type, but is REAL(4)
+  write(unit, *) 'Ok'
+
   !ERROR: If ADVANCE appears, UNIT=internal-file must not appear
   write(internal_file, advance='yes', fmt=1) 'Ok'