diff --git a/builtins_test.go b/builtins_test.go
index ee72939..6e2aa60 100644
--- a/builtins_test.go
+++ b/builtins_test.go
@@ -2,6 +2,7 @@ package fejkdata
 
 import (
 	"encoding/base64"
+	"fmt"
 	"regexp"
 	"strconv"
 	"strings"
@@ -217,8 +218,8 @@ func TestArgGuardsPanic(t *testing.T) {
 func mustPanic(t *testing.T, name string, call func()) {
 	t.Helper()
 	defer func() {
-		if recover() == nil {
-			t.Errorf("%s: no panic, want the invariant reported", name)
+		if r := recover(); r == nil || !strings.HasPrefix(fmt.Sprint(r), "fejkdata: internal error: ") {
+			t.Errorf("%s: recovered %v, want the invariant reported as an internal error", name, r)
 		}
 	}()
 	call()
diff --git a/naming_test.go b/naming_test.go
index f73e58b..b7c3e38 100644
--- a/naming_test.go
+++ b/naming_test.go
@@ -24,6 +24,27 @@ func TestNoFunctionSpellsAMethod(t *testing.T) {
 	}
 }
 
+func TestEveryPanicIsAnInternalError(t *testing.T) {
+	fset, files := sourceFiles(t)
+	for _, f := range files {
+		ast.Inspect(f, func(n ast.Node) bool {
+			call, isCall := n.(*ast.CallExpr)
+			if !isCall || !isIdent(call.Fun, "panic") {
+				return true
+			}
+			if arg, isArgCall := call.Args[0].(*ast.CallExpr); !isArgCall || !isIdent(arg.Fun, "internalError") {
+				t.Errorf("%s: panic with internalError(…), so every invariant break greps to one phrase", fset.Position(call.Pos()))
+			}
+			return true
+		})
+	}
+}
+
+func isIdent(e ast.Expr, name string) bool {
+	id, isID := e.(*ast.Ident)
+	return isID && id.Name == name
+}
+
 func namespaceFiles(t *testing.T) []*ast.File {
 	t.Helper()
 	pkg := packageDir(t)