diff --git a/modules/self-test/test_assertions.lua b/modules/self-test/test_assertions.lua
index 32f97d56..461e28cd 100644
--- a/modules/self-test/test_assertions.lua
+++ b/modules/self-test/test_assertions.lua
@@ -140,7 +140,7 @@
 			end
 		else
 			if (expected ~= actual) then
-				m.fail(depth, "expected %s but was %s", expected, actual)
+				m.fail(depth, "expected %s but was %s", expected, actual or "nil")
 			end
 		end
 		return true