diff --git a/src/pl/plpgsql/src/expected/plpgsql_call.out b/src/pl/plpgsql/src/expected/plpgsql_call.out
index 7b156f3489..1ec6182a8d 100644
--- a/src/pl/plpgsql/src/expected/plpgsql_call.out
+++ b/src/pl/plpgsql/src/expected/plpgsql_call.out
@@ -122,6 +122,18 @@ CONTEXT:  PL/pgSQL function inline_code_block line 6 at CALL
 DO
 LANGUAGE plpgsql
 $$
+DECLARE
+    x constant int := 3;
+    y int := 4;
+BEGIN
+    CALL test_proc6(2, x, y);  -- error because x is constant
+END;
+$$;
+ERROR:  variable "x" is declared CONSTANT
+CONTEXT:  PL/pgSQL function inline_code_block line 6 at CALL
+DO
+LANGUAGE plpgsql
+$$
 DECLARE
     x int := 3;
     y int := 4;
diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index ceb011810d..d523d13682 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -331,6 +331,7 @@ static void exec_prepare_plan(PLpgSQL_execstate *estate,
 static void exec_simple_check_plan(PLpgSQL_execstate *estate, PLpgSQL_expr *expr);
 static void exec_save_simple_expr(PLpgSQL_expr *expr, CachedPlan *cplan);
 static void exec_check_rw_parameter(PLpgSQL_expr *expr);
+static void exec_check_assignable(PLpgSQL_execstate *estate, int dno);
 static bool exec_eval_simple_expr(PLpgSQL_execstate *estate,
 								  PLpgSQL_expr *expr,
 								  Datum *result,
@@ -2342,9 +2343,13 @@ make_callstmt_target(PLpgSQL_execstate *estate, PLpgSQL_expr *expr)
 			if (IsA(n, Param))
 			{
 				Param	   *param = (Param *) n;
+				int			dno;
 
 				/* paramid is offset by 1 (see make_datum_param()) */
-				row->varnos[nfields++] = param->paramid - 1;
+				dno = param->paramid - 1;
+				/* must check assignability now, because grammar can't */
+				exec_check_assignable(estate, dno);
+				row->varnos[nfields++] = dno;
 			}
 			else
 			{
@@ -8242,6 +8247,43 @@ exec_check_rw_parameter(PLpgSQL_expr *expr)
 	}
 }
 
+/*
+ * exec_check_assignable --- is it OK to assign to the indicated datum?
+ *
+ * This should match pl_gram.y's check_assignable().
+ */
+static void
+exec_check_assignable(PLpgSQL_execstate *estate, int dno)
+{
+	PLpgSQL_datum *datum;
+
+	Assert(dno >= 0 && dno < estate->ndatums);
+	datum = estate->datums[dno];
+	switch (datum->dtype)
+	{
+		case PLPGSQL_DTYPE_VAR:
+		case PLPGSQL_DTYPE_PROMISE:
+		case PLPGSQL_DTYPE_REC:
+			if (((PLpgSQL_variable *) datum)->isconst)
+				ereport(ERROR,
+						(errcode(ERRCODE_ERROR_IN_ASSIGNMENT),
+						 errmsg("variable \"%s\" is declared CONSTANT",
+								((PLpgSQL_variable *) datum)->refname)));
+			break;
+		case PLPGSQL_DTYPE_ROW:
+			/* always assignable; member vars were checked at compile time */
+			break;
+		case PLPGSQL_DTYPE_RECFIELD:
+			/* assignable if parent record is */
+			exec_check_assignable(estate,
+								  ((PLpgSQL_recfield *) datum)->recparentno);
+			break;
+		default:
+			elog(ERROR, "unrecognized dtype: %d", datum->dtype);
+			break;
+	}
+}
+
 /* ----------
  * exec_set_found			Set the global found variable to true/false
  * ----------
diff --git a/src/pl/plpgsql/src/pl_gram.y b/src/pl/plpgsql/src/pl_gram.y
index 11e86c1609..62eb3f24e6 100644
--- a/src/pl/plpgsql/src/pl_gram.y
+++ b/src/pl/plpgsql/src/pl_gram.y
@@ -2098,6 +2098,12 @@ stmt_open		: K_OPEN cursor_variable
 						PLpgSQL_stmt_open *new;
 						int				  tok;
 
+						/*
+						 * cursor_variable must be assignable, since we might
+						 * change its value at runtime
+						 */
+						check_assignable((PLpgSQL_datum *) $2, @2);
+
 						new = palloc0(sizeof(PLpgSQL_stmt_open));
 						new->cmd_type = PLPGSQL_STMT_OPEN;
 						new->lineno = plpgsql_location_to_lineno(@1);
diff --git a/src/pl/plpgsql/src/sql/plpgsql_call.sql b/src/pl/plpgsql/src/sql/plpgsql_call.sql
index 8108d05060..5028398348 100644
--- a/src/pl/plpgsql/src/sql/plpgsql_call.sql
+++ b/src/pl/plpgsql/src/sql/plpgsql_call.sql
@@ -112,6 +112,18 @@ END;
 $$;
 
 
+DO
+LANGUAGE plpgsql
+$$
+DECLARE
+    x constant int := 3;
+    y int := 4;
+BEGIN
+    CALL test_proc6(2, x, y);  -- error because x is constant
+END;
+$$;
+
+
 DO
 LANGUAGE plpgsql
 $$
diff --git a/src/test/regress/expected/plpgsql.out b/src/test/regress/expected/plpgsql.out
index 33eb5e54b9..1af379d1d5 100644
--- a/src/test/regress/expected/plpgsql.out
+++ b/src/test/regress/expected/plpgsql.out
@@ -2256,6 +2256,18 @@ select refcursor_test2(20000, 20000) as "Should be false",
  f               | t
 (1 row)
 
+-- should fail to compile
+create function return_constant_refcursor() returns refcursor as $$
+declare
+    rc constant refcursor;
+begin
+    open rc for select a from rc_test;
+    return rc;
+end
+$$ language plpgsql;
+ERROR:  variable "rc" is declared CONSTANT
+LINE 5:     open rc for select a from rc_test;
+                 ^
 --
 -- tests for cursors with named parameter arguments
 --
diff --git a/src/test/regress/sql/plpgsql.sql b/src/test/regress/sql/plpgsql.sql
index bffb7b703b..da56450f0f 100644
--- a/src/test/regress/sql/plpgsql.sql
+++ b/src/test/regress/sql/plpgsql.sql
@@ -1933,6 +1933,16 @@ $$ language plpgsql;
 select refcursor_test2(20000, 20000) as "Should be false",
        refcursor_test2(20, 20) as "Should be true";
 
+-- should fail to compile
+create function return_constant_refcursor() returns refcursor as $$
+declare
+    rc constant refcursor;
+begin
+    open rc for select a from rc_test;
+    return rc;
+end
+$$ language plpgsql;
+
 --
 -- tests for cursors with named parameter arguments
 --