diff --git a/api/main_test.go b/api/main_test.go
new file mode 100644
index 0000000..b395be0
--- /dev/null
+++ b/api/main_test.go
@@ -0,0 +1,21 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestInvalidIP(t *testing.T) {
+	_, err := generateJSON("foo")
+	errwant := "invalid IP"
+	if err.Error() != errwant {
+		t.Fatalf(`generateJSON("foo") == "%v", wants "%s"`, err, errwant)
+	}
+}
+
+func TestInaccessibleIP(t *testing.T) {
+	_, err := generateJSON("0.0.0.0")
+	errwant := "unspecified IP"
+	if err.Error() != errwant {
+		t.Fatalf(`generateJSON("0.0.0.0") == "%v", wants "%s"`, err, errwant)
+	}
+}