diff --git a/flang/lib/Semantics/expression.cpp b/flang/lib/Semantics/expression.cpp
--- a/flang/lib/Semantics/expression.cpp
+++ b/flang/lib/Semantics/expression.cpp
@@ -3000,6 +3000,8 @@
             DEREF(result.GetTypeAndShape()).type(),
             ProcedureRef{std::move(proc), std::move(arguments)});
       }
+    } else {
+      Say("Function result characteristics are not known"_err_en_US);
     }
   }
   return std::nullopt;
diff --git a/flang/test/Semantics/resolve09.f90 b/flang/test/Semantics/resolve09.f90
--- a/flang/test/Semantics/resolve09.f90
+++ b/flang/test/Semantics/resolve09.f90
@@ -18,6 +18,7 @@
   !ERROR: Cannot call function 'f' like a subroutine
   call f
   !ERROR: Cannot call subroutine 's' like a function
+  !ERROR: Function result characteristics are not known
   i = s()
 contains
   function f()