diff --git a/lib/reader.c b/lib/reader.c
index 6bdd650..1f73116 100644
--- a/lib/reader.c
+++ b/lib/reader.c
@@ -123,6 +123,9 @@ const expression_t *
 read_expression(memory_pool_t *pool, const char *input, int len)
 {
 	expression_t *expression;
-	parse_expression(pool, input, len, &expression);
-	return expression;
+	const int used = parse_expression(pool, input, len, &expression);
+	if (used == len)
+		return expression;
+	else
+		return NULL;
 }
diff --git a/tests/reader_tests.c b/tests/reader_tests.c
index 6e7e4e6..8a4b238 100644
--- a/tests/reader_tests.c
+++ b/tests/reader_tests.c
@@ -166,6 +166,20 @@ static void paren_1_plus_2_times_3_close_paren_parses_as_left_nested(void)
 	ASSERT_EQUAL(3, operand1->number);
 }
 
+static void garbage_input_yeilds_null(void)
+{
+	init_memory_pool(&pool);
+	const expression_t *result = read_expression(&pool, "arwizxhu", 8);
+	ASSERT_NULL(result);
+}
+
+static void valid_expression_followed_by_garbage_yeilds_null(void)
+{
+	init_memory_pool(&pool);
+	const expression_t *result = read_expression(&pool, "5+2jvkre", 8);
+	ASSERT_NULL(result);
+}
+
 int main(void)
 {
 	TESTING_BEGIN();
@@ -180,5 +194,7 @@ int main(void)
 	RUN_TEST(input_1_times_2_times_3_is_parsed_as_a_left_nested_application);
 	RUN_TEST(input_1_plus_2_times_3_is_parsed_as_a_right_nested_application);
 	RUN_TEST(paren_1_plus_2_times_3_close_paren_parses_as_left_nested);
+	RUN_TEST(garbage_input_yeilds_null);
+	RUN_TEST(valid_expression_followed_by_garbage_yeilds_null);
 	TESTING_END();
 }