diff --git a/builtins.go b/builtins.go
index dd8e72d..31c3916 100644
--- a/builtins.go
+++ b/builtins.go
@@ -115,7 +115,7 @@ func formatFloat(v float64, dp int) string {
 func atoi(s string) int {
 	n, err := strconv.Atoi(s)
 	if err != nil {
-		panic(fmt.Sprintf("fejkdata: builtin arg %q reached prep unvalidated: %v", s, err))
+		panic(internalError("builtin arg %q reached prep unvalidated: %v", s, err))
 	}
 	return n
 }
@@ -124,7 +124,7 @@ func atoi(s string) int {
 func atof(s string) float64 {
 	f, err := strconv.ParseFloat(s, 64)
 	if err != nil {
-		panic(fmt.Sprintf("fejkdata: builtin arg %q reached prep unvalidated: %v", s, err))
+		panic(internalError("builtin arg %q reached prep unvalidated: %v", s, err))
 	}
 	return f
 }
diff --git a/calc.go b/calc.go
index 60801fb..827fde1 100644
--- a/calc.go
+++ b/calc.go
@@ -38,7 +38,7 @@ func (n calcIdx) eval(operands []string) float64 {
 // than returning NaN, so a node kind indexVars forgets is a stack trace and not a
 // silently wrong number.
 func (n calcVar) eval([]string) float64 {
-	panic(fmt.Sprintf("fejkdata: calc operand %q was never placed", string(n)))
+	panic(internalError("calc operand %q was never placed", string(n)))
 }
 
 func (n calcNeg) eval(operands []string) float64 { return -n.x.eval(operands) }
@@ -191,7 +191,7 @@ func neverNumeric(n node) (text string, never bool) {
 func calcPrep(args []string) callFn {
 	expr, err := parseCalc(args[0])
 	if err != nil { // a nil AST would be a nil dereference per render, with no message
-		panic(fmt.Sprintf("fejkdata: calc(%q) reached prep unparsed: %v", args[0], err))
+		panic(internalError("calc(%q) reached prep unparsed: %v", args[0], err))
 	}
 	at := make(map[string]int)
 	for i, name := range calcVars(expr) {
@@ -219,7 +219,7 @@ func indexVars(n calcNode, at map[string]int) calcNode {
 	case calcVar:
 		i, placed := at[string(n)]
 		if !placed { // calcVars named every operand, so a miss means the two disagree
-			panic(fmt.Sprintf("fejkdata: calc operand %q is not among the names read for it", string(n)))
+			panic(internalError("calc operand %q is not among the names read for it", string(n)))
 		}
 		return calcIdx(i)
 	case calcNeg:
diff --git a/fejkdata.go b/fejkdata.go
index 7999c23..25c0666 100644
--- a/fejkdata.go
+++ b/fejkdata.go
@@ -240,3 +240,7 @@ func newRand(seed uint64, seeded bool) (*session, error) {
 	}
 	return &session{Rand: r, counters: map[string]uint64{}}, nil
 }
+
+func internalError(format string, a ...any) string {
+	return "fejkdata: internal error: " + fmt.Sprintf(format, a...)
+}
diff --git a/graph.go b/graph.go
index 10d7e65..7d30b9e 100644
--- a/graph.go
+++ b/graph.go
@@ -162,7 +162,7 @@ func renderEdges(n node) []renderEdge {
 	case *folder, *nullItem:
 		return nil
 	default:
-		panic(fmt.Sprintf("fejkdata: renderEdges has no case for node %T", n))
+		panic(internalError("renderEdges has no case for node %T", n))
 	}
 }
 
diff --git a/hold.go b/hold.go
index 0b30533..590ee5b 100644
--- a/hold.go
+++ b/hold.go
@@ -1,9 +1,6 @@
 package fejkdata
 
-import (
-	"fmt"
-	"strings"
-)
+import "strings"
 
 // hold is what one expansion has drawn for the names it holds: the variant each was drawn as, so
 // every path under it reads one variant; and the value each read produced, by its path, so the
@@ -39,7 +36,7 @@ func readField(s *session, t *template, held *hold, sc renderScope, a arm) readV
 	}
 	if !t.compiled.held[a.key] {
 		if len(a.tail) > 0 {
-			panic(fmt.Sprintf("fejkdata: %q reads a path into %q, which the expansion does not hold", a.spelling, a.key))
+			panic(internalError("%q reads a path into %q, which the expansion does not hold", a.spelling, a.key))
 		}
 		return readValue{text: render(s, t.head(a.key), sc)}
 	}
diff --git a/holdset.go b/holdset.go
index 810b8d2..d0a2e0c 100644
--- a/holdset.go
+++ b/holdset.go
@@ -1,9 +1,6 @@
 package fejkdata
 
-import (
-	"fmt"
-	"strings"
-)
+import "strings"
 
 // holdSet is one render's reference draws: the unnamed draw group's, and each named one's; and a
 // test's trace of its reads.
@@ -61,7 +58,7 @@ func (sc renderScope) at(n node, pins *pinSet) renderScope {
 // rowOf is the row of t its columns render from.
 func (sc renderScope) rowOf(t *table) int {
 	if sc.row.t != t {
-		panic(fmt.Sprintf("fejkdata: a column of %s is rendered with no row pinned", t.category))
+		panic(internalError("a column of %s is rendered with no row pinned", t.category))
 	}
 	return sc.row.row
 }
diff --git a/layout.go b/layout.go
index 69f26c1..af98d7e 100644
--- a/layout.go
+++ b/layout.go
@@ -54,7 +54,7 @@ func layoutArg(a string) (string, error) {
 func layoutOf(a string) string {
 	layout, err := layoutArg(a)
 	if err != nil {
-		panic(fmt.Sprintf("fejkdata: builtin arg %q reached prep unvalidated: %v", a, err))
+		panic(internalError("builtin arg %q reached prep unvalidated: %v", a, err))
 	}
 	return layout
 }
@@ -128,11 +128,11 @@ func timeArg(_ map[string]node, a []string) error {
 func datePrep(a []string) callFn {
 	from, err := time.Parse(dayLayout, a[0])
 	if err != nil {
-		panic(fmt.Sprintf("fejkdata: builtin arg %q reached prep unvalidated: %v", a[0], err))
+		panic(internalError("builtin arg %q reached prep unvalidated: %v", a[0], err))
 	}
 	to, err := time.Parse(dayLayout, a[1])
 	if err != nil {
-		panic(fmt.Sprintf("fejkdata: builtin arg %q reached prep unvalidated: %v", a[1], err))
+		panic(internalError("builtin arg %q reached prep unvalidated: %v", a[1], err))
 	}
 	layout, start, span := layoutOf(a[2]), from.Unix(), int(to.Unix()-from.Unix())+86400
 	return func(s *session, _ string, _ []string) string {
diff --git a/path.go b/path.go
index 3fd8b5b..771248c 100644
--- a/path.go
+++ b/path.go
@@ -353,7 +353,7 @@ type pathDraw struct {
 func drawPath(n node, tail []string, level string, w *pathDraw) node {
 	leaf, err := w.walk(n, tail)
 	if err != nil {
-		panic(fmt.Sprintf("fejkdata: %s: %v; the path should have been proved before it was drawn", join(level, joinSegments(tail)), err))
+		panic(internalError("%s: %v; the path should have been proved before it was drawn", join(level, joinSegments(tail)), err))
 	}
 	return leaf
 }
diff --git a/pins.go b/pins.go
index f16cee0..5866e3b 100644
--- a/pins.go
+++ b/pins.go
@@ -32,7 +32,7 @@ func (p *pinSet) pinned(t *table) (int, bool) {
 func (p *pinSet) mustRow(t *table) int {
 	r, ok := p.pinned(t)
 	if !ok {
-		panic(fmt.Sprintf("fejkdata: a column of %s is rendered with no row pinned", t.category))
+		panic(internalError("a column of %s is rendered with no row pinned", t.category))
 	}
 	return r
 }
diff --git a/render.go b/render.go
index b6490d6..739ce15 100644
--- a/render.go
+++ b/render.go
@@ -85,7 +85,7 @@ func render(s *session, n node, sc renderScope) string {
 		}
 		return b.String()
 	default:
-		panic(fmt.Sprintf("fejkdata: uncompiled node %T", n))
+		panic(internalError("uncompiled node %T", n))
 	}
 }
 
diff --git a/struct.go b/struct.go
index 67f5643..a6f3df1 100644
--- a/struct.go
+++ b/struct.go
@@ -422,6 +422,6 @@ func setText(field reflect.Value, text string) {
 		field.SetUint(n)
 	}
 	if err != nil {
-		panic(fmt.Sprintf("fejkdata: %q reached a %s field unproven: %v", text, field.Type(), err))
+		panic(internalError("%q reached a %s field unproven: %v", text, field.Type(), err))
 	}
 }
diff --git a/transform.go b/transform.go
index d5a3ae6..e5f965e 100644
--- a/transform.go
+++ b/transform.go
@@ -17,7 +17,7 @@ var transforms = map[string]func(string) string{
 func withTransforms(calls map[string]builtin) map[string]builtin {
 	for name, fn := range transforms {
 		if _, clash := calls[name]; clash {
-			panic("fejkdata: " + name + " is registered as a builtin and a transform")
+			panic(internalError("%s is registered as a builtin and a transform", name))
 		}
 		calls[name] = builtin{arity: 1, checkArgs: transformArg, prep: transformPrep(fn), operands: transformOperand}
 	}
@@ -68,7 +68,7 @@ func transformPrep(outer func(string) string) func([]string) callFn {
 	return func(a []string) callFn {
 		_, chain, err := unwrapTransform(a[0])
 		if err != nil {
-			panic(fmt.Sprintf("fejkdata: transform arg %q reached prep unvalidated: %v", a[0], err))
+			panic(internalError("transform arg %q reached prep unvalidated: %v", a[0], err))
 		}
 		return func(_ *session, _ string, operands []string) string {
 			v := operands[0]
diff --git a/value.go b/value.go
index 3a764d8..0c73593 100644
--- a/value.go
+++ b/value.go
@@ -176,7 +176,7 @@ func (p *valueProof) proveTemplate(t *template) proven {
 func (p *valueProof) proveCalc(t *template, body string, args []string) proven {
 	expr, err := parseCalc(args[0])
 	if err != nil {
-		panic(fmt.Sprintf("fejkdata: calc(%q) reached a proof unparsed: %v", args[0], err))
+		panic(internalError("calc(%q) reached a proof unparsed: %v", args[0], err))
 	}
 	v, doubt := p.proveExpr(expr, t.fields)
 	if doubt == "" && !(magnitude(v) <= calcLimit) {
@@ -219,7 +219,7 @@ func (p *valueProof) proveExpr(n calcNode, fields map[string]node) (proven, stri
 		}
 		return combine(n, l, r)
 	}
-	panic(fmt.Sprintf("fejkdata: calc node %T has no bound", n))
+	panic(internalError("calc node %T has no bound", n))
 }
 
 // combine bounds one operation from the bounds of its sides.