diff --git a/testing/errors/subst6.a b/testing/errors/subst6.a
new file mode 100644
index 0000000..93e0e77
--- /dev/null
+++ b/testing/errors/subst6.a
@@ -0,0 +1,5 @@
+	*=$1000
+	!set inner = 6
+	!set outer = 7
+	; substitutions cannot be nested
+base?(?(inner)outer)	nop	; -> "no string given", "syntax error"
diff --git a/testing/errors/subst7.a b/testing/errors/subst7.a
new file mode 100644
index 0000000..2cf29c3
--- /dev/null
+++ b/testing/errors/subst7.a
@@ -0,0 +1,5 @@
+	*=$1000
+	!set inner = 6
+	!set outer = "?(inner)"
+	; substitution results are not substituted again
+base?(outer)	nop	; -> "syntax error"