diff --git a/stack/cas/cassession2.class.php b/stack/cas/cassession2.class.php
index 9bdec031de..5805b89684 100644
--- a/stack/cas/cassession2.class.php
+++ b/stack/cas/cassession2.class.php
@@ -410,6 +410,10 @@ public function instantiate(): bool {
                             $asts[$key] = $value;
                         }
                     } catch (Exception $e) {
+                        // TODO: issue #1279 would change this exception to add in an error associated
+                        // with the values collected rather than a stack_exception.
+                        // We would then add something like this to allow the process to continue.
+                        // $asts[$key] = maxima_parser_utils::parse('null', 'Root', false);
                         throw new stack_exception('stack_cas_session: tried to parse the value ' .
                                 $value . ', but got the following exception ' . $e->getMessage());
                     }
diff --git a/tests/caskeyval_exception_test.php b/tests/caskeyval_exception_test.php
index 10500f61f4..fc12f609d9 100644
--- a/tests/caskeyval_exception_test.php
+++ b/tests/caskeyval_exception_test.php
@@ -65,4 +65,18 @@ public function test_exception_7() {
         $this->expectException(stack_exception::class);
         $at1 = new stack_cas_keyval('x=1', 't', false);
     }
+
+    public function test_stack_compile_unexpected_lambda() {
+        $this->expectException(stack_exception::class);
+        // This is related to issue #1279.
+        $tests = 'a:b+1; c:a-a(d+1);';
+        $kv = new stack_cas_keyval($tests);
+        $this->asserttrue($kv->get_valid());
+        $expected = [];
+        $this->assertEquals($expected, $kv->get_errors());
+        $kv->instantiate();
+        $s = $kv->get_session();
+        $s->instantiate();
+        $this->assertEquals($s->get_by_key('c')->get_evaluationform(), 'c:(b+1)-(b+1)(d+1)');
+    }
 }
diff --git a/tests/caskeyval_test.php b/tests/caskeyval_test.php
index f8a5a1dcbb..1ab8874b92 100644
--- a/tests/caskeyval_test.php
+++ b/tests/caskeyval_test.php
@@ -335,8 +335,6 @@ public function test_stack_compile() {
                  'q:matrix([i],[j],[k]);v:dotproduct(p,q);';
         $kv = new stack_cas_keyval($tests);
         $this->assertTrue($kv->get_valid());
-        $expected = [
-        ];
         $compiled = $kv->compile('kv-test');
 
         $expected = '(_EC(errcatch(stack_reset_vars(true)),"kv-test/1:1-1:2"),' .
@@ -356,8 +354,6 @@ public function test_stack_compile_preamble_end1() {
             'q:matrix([i],[j],[k]);v:dotproduct(p,q);';
         $kv = new stack_cas_keyval($tests);
         $this->assertTrue($kv->get_valid());
-        $expected = [
-        ];
         $compiled = $kv->compile('kv-test');
 
         $expected = '(_EC(errcatch(stack_reset_vars(true)),"kv-test/1:1-1:2"),' .
@@ -370,4 +366,15 @@ public function test_stack_compile_preamble_end1() {
         $expected = '(_EC(errcatch(n1:1),"kv-test/1:24-1:2"),true)';
         $this->assertEquals($expected, $compiled['contextvariables']);
     }
+
+    public function test_stack_compile_unexpected_lambda() {
+        // This is related to issue #1279.
+        $tests = 'c:(b+1)-(b+1)(d+1);';
+        $kv = new stack_cas_keyval($tests);
+        $this->assertfalse($kv->get_valid());
+        $expected = ['You seem to be missing * characters. Perhaps you meant to type ' .
+            '<span class="stacksyntaxexample">c:(b+1)-(b+1)<span class="stacksyntaxexamplehighlight">' .
+            '*</span>(d+1)</span>.'];
+        $this->assertEquals($expected, $kv->get_errors());
+    }
 }
diff --git a/tests/castext_test.php b/tests/castext_test.php
index bfbee13d4e..66f49baec8 100644
--- a/tests/castext_test.php
+++ b/tests/castext_test.php
@@ -2451,4 +2451,28 @@ public function test_make_mult_sgn_stackunits() {
         $this->assertEquals('\({10\, \frac{m}{s}}\), \({1\cdot s^ {- 1 }}\), \({1\, s^ {- 1 }}\). ' .
             'Multiplication unaffected: \({a\cdot b}\).', $at1->get_rendered());
     }
+
+    /**
+     * @covers \qtype_stack\stack_cas_castext2_latex
+     * @covers \qtype_stack\stack_cas_keyval
+     */
+    public function test_unexpected_lambda() {
+        $a2 = ['a:b+1', 'c:a-a(d+1)'];
+        $s2 = [];
+        foreach ($a2 as $s) {
+            $cs = stack_ast_container::make_from_teacher_source($s, '', new stack_cas_security(), []);
+            $this->assertTrue($cs->get_valid());
+            $s2[] = $cs;
+        }
+        $options = new stack_options();
+        $options->set_option('simplify', false);
+        $cs2 = new stack_cas_session2($s2, $options, 0);
+        $at1 = castext2_evaluatable::make_from_source('{@c@}',
+            'test-case');
+        $this->assertTrue($at1->get_valid());
+        $cs2->add_statement($at1);
+        $cs2->instantiate();
+        $expected = '\({b+1-\left(b+1\right)(d+1)}\)';
+        $this->assertEquals($expected, $at1->get_rendered());
+    }
 }