diff --git a/conformance/system_macros/sum.ion b/conformance/system_macros/sum.ion
index 085e53e..68e0b00 100644
--- a/conformance/system_macros/sum.ion
+++ b/conformance/system_macros/sum.ion
@@ -17,7 +17,7 @@
                (produces 0)))
 
 (ion_1_1 "sum produces a single, unannotated integer that is the sum of"
-         (each "two integers"
+         (each "integers"
                (text "(:sum -1  4)")
                (text "(:sum  0  3)")
                (text "(:sum  1  2)")
@@ -28,12 +28,13 @@
                (text "(:sum a::1 2)")
                (text "(:sum 1 b::2)")
                (text "(:sum a::1 b::2)")
-               // Values that produce an integer
+               "expressions that produce integers"
                (text "(:sum (:values 1) 2)")
                (text "(:sum 1 (:values 2))")
-               // ...including other sums
+               "other sums"
                (text "(:sum (:sum 1 1) 1)")
                (text "(:sum 1 (:sum 1 1))")
+               (text "(:sum (:sum -4 3) (:sum 2 2))")
                (produces 3)))
 
 (ion_1_1 "sum is commutative"
@@ -45,7 +46,12 @@
          (each (text "(:sum -2  4)") (text "(:sum  4 -2)") (produces  2)))
 
 (ion_1_1 "arguments may not be"
-         (each "any null"
+         (each "less than two integers"
+               (text "(:sum)")
+               (text "(:sum 1)")
+               "more than two integers"
+               (text "(:sum 1 2 3)")
+               "any null"
                (text "(:sum 1 null)")
                (text "(:sum 1 null.int)")
                (text "(:sum 1 null.float)")
@@ -73,4 +79,6 @@
                (text '''(:sum 1 '3')''')
                (text '''(:sum "3" 2)''')
                (text '''(:sum '3' 2)''')
+               "a single expression that produces two integers"
+               (text "(:sum (:values 1 3))")
                (signals "invalid argument")))