diff --git a/builtins_test.go b/builtins_test.go
index 52ba4f4..9be496e 100644
--- a/builtins_test.go
+++ b/builtins_test.go
@@ -8,6 +8,8 @@ import (
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/larvit/fejkdata/internal/invariant"
 )
 
 func TestBuiltinIDGenerators(t *testing.T) {
@@ -188,12 +190,12 @@ func ibanValid(s string) bool {
 	return rem == 1
 }
 
-// mustPanic fails unless call panics with internalError's phrase, which is what
+// mustPanic fails unless call panics with invariant.Broken's phrase, which is what
 // separates a reported invariant break from a silently wrong value.
 func mustPanic(t *testing.T, name string, call func()) {
 	t.Helper()
 	defer func() {
-		if r := recover(); r == nil || !strings.HasPrefix(fmt.Sprint(r), "fejkdata: internal error: ") {
+		if r := recover(); r == nil || !strings.HasPrefix(fmt.Sprint(r), invariant.Broken("")) {
 			t.Errorf("%s: recovered %v, want the invariant reported as an internal error", name, r)
 		}
 	}()
diff --git a/internal/builtinfunc/builtinfunc_test.go b/internal/builtinfunc/builtinfunc_test.go
index 140b0f1..7a3d5dc 100644
--- a/internal/builtinfunc/builtinfunc_test.go
+++ b/internal/builtinfunc/builtinfunc_test.go
@@ -7,6 +7,7 @@ import (
 	"testing"
 
 	"github.com/larvit/fejkdata/internal/grammar"
+	"github.com/larvit/fejkdata/internal/invariant"
 )
 
 // TestRegistryShapes pins the builtin contract Prep relies on: every entry supplies prep,
@@ -67,12 +68,12 @@ func TestArgGuardsPanic(t *testing.T) {
 	}
 }
 
-// mustPanic fails unless call panics with internalError's phrase, which is what
+// mustPanic fails unless call panics with invariant.Broken's phrase, which is what
 // separates a reported invariant break from a silently wrong value.
 func mustPanic(t *testing.T, name string, call func()) {
 	t.Helper()
 	defer func() {
-		if r := recover(); r == nil || !strings.HasPrefix(fmt.Sprint(r), "fejkdata: internal error: ") {
+		if r := recover(); r == nil || !strings.HasPrefix(fmt.Sprint(r), invariant.Broken("")) {
 			t.Errorf("%s: recovered %v, want the invariant reported as an internal error", name, r)
 		}
 	}()
diff --git a/internal/datatype/datatype_test.go b/internal/datatype/datatype_test.go
index ec1d1f3..d53d9fe 100644
--- a/internal/datatype/datatype_test.go
+++ b/internal/datatype/datatype_test.go
@@ -23,3 +23,9 @@ func TestStringNamesADatatypeNoDataWrites(t *testing.T) {
 		t.Errorf("String = %q, want DataType(9)", got)
 	}
 }
+
+func TestNounNamesADatatypeNoDataWrites(t *testing.T) {
+	if got := Noun(DataType(9)); got != "DataType(9)" {
+		t.Errorf("Noun = %q, want DataType(9)", got)
+	}
+}
diff --git a/internal/proven/proven_test.go b/internal/proven/proven_test.go
index db75ece..6f5ec11 100644
--- a/internal/proven/proven_test.go
+++ b/internal/proven/proven_test.go
@@ -20,7 +20,7 @@ func TestLiteralSaysWhichDatatypesItsTextIs(t *testing.T) {
 		{"-0", [datatype.Count]bool{datatype.Integer: true, datatype.Number: true, datatype.Boolean: true}},
 	} {
 		v := Literal(c.text)
-		for d := datatype.Integer; d <= datatype.Boolean; d++ {
+		for d := datatype.Integer; d < datatype.Count; d++ {
 			if (v.Not[d] != "") != c.not[d] {
 				t.Errorf("Literal(%q).Not[%s] = %q", c.text, d, v.Not[d])
 			}