diff --git a/runtime/sam/expr/ztests/cast-bytes-string-err.yaml b/runtime/sam/expr/ztests/cast-bytes-string-err.yaml
deleted file mode 100644
index c3e5eb69b8..0000000000
--- a/runtime/sam/expr/ztests/cast-bytes-string-err.yaml
+++ /dev/null
@@ -1,7 +0,0 @@
-spq: 'put s:=b::string'
-
-input: |
-  {b:0xc328}
-
-output: |
-  {b:0xc328,s:error({message:"cannot cast to string: invalid UTF-8",on:0xc328})}
diff --git a/runtime/vam/expr/cast/bool.go b/runtime/vam/expr/cast/bool.go
index 5c553b7599..e2404d1259 100644
--- a/runtime/vam/expr/cast/bool.go
+++ b/runtime/vam/expr/cast/bool.go
@@ -6,7 +6,7 @@ import (
 	"github.com/brimdata/super/vector/bitvec"
 )
 
-func castToBool(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
+func castToBool(vec vector.Any, index []uint32) (vector.Any, []uint32, string, bool) {
 	var out *vector.Bool
 	switch vec := vec.(type) {
 	case *vector.Int:
@@ -17,9 +17,9 @@ func castToBool(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
 		out = numberToBool(vec.Values, index)
 	case *vector.String:
 		vvec, errs := stringToBool(vec, index)
-		return vvec, errs, true
+		return vvec, errs, "", true
 	default:
-		return nil, nil, false
+		return nil, nil, "", false
 	}
 	nulls := vector.NullsOf(vec)
 	if index == nil {
@@ -27,7 +27,7 @@ func castToBool(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
 	} else {
 		out.Nulls = nulls.Pick(index)
 	}
-	return out, nil, true
+	return out, nil, "", true
 }
 
 func numberToBool[E numeric](s []E, index []uint32) *vector.Bool {
diff --git a/runtime/vam/expr/cast/bytes.go b/runtime/vam/expr/cast/bytes.go
index d1fa2dc2af..3cea7f3510 100644
--- a/runtime/vam/expr/cast/bytes.go
+++ b/runtime/vam/expr/cast/bytes.go
@@ -4,14 +4,14 @@ import (
 	"github.com/brimdata/super/vector"
 )
 
-func castToBytes(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
+func castToBytes(vec vector.Any, index []uint32) (vector.Any, []uint32, string, bool) {
 	strVec, ok := vec.(*vector.String)
 	if !ok {
-		return nil, nil, false
+		return nil, nil, "", false
 	}
 	out := vector.Any(vector.NewBytes(strVec.Table(), vector.NullsOf(strVec)))
 	if index != nil {
 		out = vector.Pick(out, index)
 	}
-	return out, nil, true
+	return out, nil, "", true
 }
diff --git a/runtime/vam/expr/cast/cast.go b/runtime/vam/expr/cast/cast.go
index 265525613a..109999ed45 100644
--- a/runtime/vam/expr/cast/cast.go
+++ b/runtime/vam/expr/cast/cast.go
@@ -13,7 +13,7 @@ func To(sctx *super.Context, vec vector.Any, typ super.Type) vector.Any {
 	var c caster
 	id := typ.ID()
 	if super.IsNumber(id) {
-		c = func(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
+		c = func(vec vector.Any, index []uint32) (vector.Any, []uint32, string, bool) {
 			return castToNumber(vec, typ, index)
 		}
 	} else {
@@ -29,29 +29,30 @@ func To(sctx *super.Context, vec vector.Any, typ super.Type) vector.Any {
 		case super.IDNet:
 			c = castToNet
 		case super.IDType:
-			c = func(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
+			c = func(vec vector.Any, index []uint32) (vector.Any, []uint32, string, bool) {
 				return castToType(sctx, vec, index)
 			}
 		default:
-			return errCastFailed(sctx, vec, typ)
+			return errCastFailed(sctx, vec, typ, "")
 		}
 	}
 	return assemble(sctx, vec, typ, c)
 }
 
-type caster func(vector.Any, []uint32) (vector.Any, []uint32, bool)
+type caster func(vector.Any, []uint32) (vector.Any, []uint32, string, bool)
 
 func assemble(sctx *super.Context, vec vector.Any, typ super.Type, fn caster) vector.Any {
 	var out vector.Any
 	var errs []uint32
+	var errMsg string
 	var ok bool
 	switch vec := vec.(type) {
 	case *vector.Const:
 		return castConst(sctx, vec, typ)
 	case *vector.View:
-		out, errs, ok = fn(vec.Any, vec.Index)
+		out, errs, errMsg, ok = fn(vec.Any, vec.Index)
 	case *vector.Dict:
-		out, errs, ok = fn(vec.Any, nil)
+		out, errs, errMsg, ok = fn(vec.Any, nil)
 		if ok {
 			if len(errs) > 0 {
 				index, counts, nulls, nerrs := vec.RebuildDropTags(errs...)
@@ -62,13 +63,13 @@ func assemble(sctx *super.Context, vec vector.Any, typ super.Type, fn caster) ve
 			}
 		}
 	default:
-		out, errs, ok = fn(vec, nil)
+		out, errs, errMsg, ok = fn(vec, nil)
 	}
 	if !ok {
-		return errCastFailed(sctx, vec, typ)
+		return errCastFailed(sctx, vec, typ, errMsg)
 	}
 	if len(errs) > 0 {
-		return vector.Combine(out, errs, errCastFailed(sctx, vector.Pick(vec, errs), typ))
+		return vector.Combine(out, errs, errCastFailed(sctx, vector.Pick(vec, errs), typ, errMsg))
 	}
 	return out
 }
@@ -88,17 +89,21 @@ func castConst(sctx *super.Context, vec *vector.Const, typ super.Type) vector.An
 					trueCount++
 				}
 			}
-			err := errCastFailed(sctx, vector.NewConst(vec.Value(), vec.Len()-trueCount, bitvec.Zero), typ)
+			err := errCastFailed(sctx, vector.NewConst(vec.Value(), vec.Len()-trueCount, bitvec.Zero), typ, "")
 			nulls := vector.NewConst(super.NewValue(typ, nil), trueCount, bitvec.Zero)
 			return vector.NewDynamic(index, []vector.Any{err, nulls})
 		}
-		return errCastFailed(sctx, vec, typ)
+		return errCastFailed(sctx, vec, typ, "")
 	}
 	return vector.NewConst(val, vec.Len(), vec.Nulls)
 }
 
-func errCastFailed(sctx *super.Context, vec vector.Any, typ super.Type) vector.Any {
-	return vector.NewWrappedError(sctx, "cannot cast to "+sup.FormatType(typ), vec)
+func errCastFailed(sctx *super.Context, vec vector.Any, typ super.Type, msgSuffix string) vector.Any {
+	msg := "cannot cast to " + sup.FormatType(typ)
+	if msgSuffix != "" {
+		msg = msg + ": " + msgSuffix
+	}
+	return vector.NewWrappedError(sctx, msg, vec)
 }
 
 func lengthOf(vec vector.Any, index []uint32) uint32 {
diff --git a/runtime/vam/expr/cast/ip.go b/runtime/vam/expr/cast/ip.go
index cb7ebbacee..af722e208e 100644
--- a/runtime/vam/expr/cast/ip.go
+++ b/runtime/vam/expr/cast/ip.go
@@ -8,10 +8,10 @@ import (
 	"github.com/brimdata/super/vector/bitvec"
 )
 
-func castToIP(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
+func castToIP(vec vector.Any, index []uint32) (vector.Any, []uint32, string, bool) {
 	switch vec := vec.(type) {
 	case *vector.IP:
-		return vec, nil, true
+		return vec, nil, "", true
 	case *vector.String:
 		n := lengthOf(vec, index)
 		var nulls bitvec.Bits
@@ -37,16 +37,16 @@ func castToIP(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
 			}
 			ips = append(ips, ip)
 		}
-		return vector.NewIP(ips, nulls), errs, true
+		return vector.NewIP(ips, nulls), errs, "", true
 	default:
-		return nil, nil, false
+		return nil, nil, "", false
 	}
 }
 
-func castToNet(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
+func castToNet(vec vector.Any, index []uint32) (vector.Any, []uint32, string, bool) {
 	switch vec := vec.(type) {
 	case *vector.Net:
-		return vec, nil, true
+		return vec, nil, "", true
 	case *vector.String:
 		n := lengthOf(vec, index)
 		var nulls bitvec.Bits
@@ -72,8 +72,8 @@ func castToNet(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
 			}
 			nets = append(nets, net)
 		}
-		return vector.NewNet(nets, nulls), errs, true
+		return vector.NewNet(nets, nulls), errs, "", true
 	default:
-		return nil, nil, false
+		return nil, nil, "", false
 	}
 }
diff --git a/runtime/vam/expr/cast/number.go b/runtime/vam/expr/cast/number.go
index f269668d69..3858242649 100644
--- a/runtime/vam/expr/cast/number.go
+++ b/runtime/vam/expr/cast/number.go
@@ -17,10 +17,10 @@ type numeric interface {
 	constraints.Float | constraints.Integer
 }
 
-func castToNumber(vec vector.Any, typ super.Type, index []uint32) (vector.Any, []uint32, bool) {
+func castToNumber(vec vector.Any, typ super.Type, index []uint32) (vector.Any, []uint32, string, bool) {
 	if vec.Type().ID() == super.IDString {
 		out, errs := castStringToNumber(vec, typ, index)
-		return out, errs, true
+		return out, errs, "", true
 	}
 	nulls := vector.NullsOf(vec)
 	if index != nil {
@@ -32,21 +32,21 @@ func castToNumber(vec vector.Any, typ super.Type, index []uint32) (vector.Any, [
 		if len(errs) > 0 {
 			nulls = nulls.Pick(inverseIndex(errs, nulls.Len()))
 		}
-		return vector.NewInt(typ, vals, nulls), errs, true
+		return vector.NewInt(typ, vals, nulls), errs, "", true
 	case super.IsUnsigned(id):
 		vals, errs := toNumeric[uint64](vec, typ, index)
 		if len(errs) > 0 {
 			nulls = nulls.Pick(inverseIndex(errs, nulls.Len()))
 		}
-		return vector.NewUint(typ, vals, nulls), errs, true
+		return vector.NewUint(typ, vals, nulls), errs, "", true
 	case super.IsFloat(id):
 		vals, errs := toNumeric[float64](vec, typ, index)
 		if errs != nil {
 			nulls = nulls.Pick(inverseIndex(errs, nulls.Len()))
 		}
-		return vector.NewFloat(typ, vals, nulls), errs, true
+		return vector.NewFloat(typ, vals, nulls), errs, "", true
 	default:
-		return nil, nil, false
+		return nil, nil, "", false
 	}
 }
 
diff --git a/runtime/vam/expr/cast/string.go b/runtime/vam/expr/cast/string.go
index 4c27b85a9c..6b24a41539 100644
--- a/runtime/vam/expr/cast/string.go
+++ b/runtime/vam/expr/cast/string.go
@@ -3,6 +3,7 @@ package cast
 import (
 	"strconv"
 	"time"
+	"unicode/utf8"
 
 	"github.com/brimdata/super"
 	"github.com/brimdata/super/pkg/nano"
@@ -11,7 +12,7 @@ import (
 	"github.com/brimdata/super/vector"
 )
 
-func castToString(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
+func castToString(vec vector.Any, index []uint32) (vector.Any, []uint32, string, bool) {
 	nulls := vector.NullsOf(vec)
 	if index != nil {
 		nulls = nulls.Pick(index)
@@ -56,21 +57,35 @@ func castToString(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
 		}
 	case *vector.String:
 		if index == nil {
-			return vec, nil, true
+			return vec, nil, "", true
 		}
 		for _, idx := range index {
 			bytes = append(bytes, vec.Value(idx)...)
 			offs = append(offs, uint32(len(bytes)))
 		}
 	case *vector.Bytes:
+		var errs []uint32
 		for i := range n {
 			idx := i
 			if index != nil {
 				idx = index[i]
 			}
-			bytes = append(bytes, vec.Value(idx)...)
-			offs = append(offs, uint32(len(bytes)))
+			if !utf8.Valid(vec.Value(idx)) {
+				errs = append(errs, i)
+			}
+		}
+		const errMsg = "invalid UTF-8"
+		if len(errs) == int(n) {
+			return nil, nil, errMsg, false
+		}
+		out := vector.Any(vector.NewString(vec.Table(), vec.Nulls))
+		if index != nil {
+			out = vector.Pick(out, index)
+		}
+		if len(errs) > 0 {
+			out = vector.ReversePick(out, errs)
 		}
+		return out, errs, errMsg, true
 	case *vector.IP:
 		for i := range n {
 			idx := i
@@ -115,7 +130,7 @@ func castToString(vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
 			offs = append(offs, uint32(len(bytes)))
 		}
 	}
-	return vector.NewString(vector.NewBytesTable(offs, bytes), nulls), nil, true
+	return vector.NewString(vector.NewBytesTable(offs, bytes), nulls), nil, "", true
 }
 
 func timeToString(vec *vector.Int, index []uint32, n uint32) ([]uint32, []byte) {
diff --git a/runtime/vam/expr/cast/type.go b/runtime/vam/expr/cast/type.go
index 0070eb18d8..5d8f5fa2b7 100644
--- a/runtime/vam/expr/cast/type.go
+++ b/runtime/vam/expr/cast/type.go
@@ -7,10 +7,10 @@ import (
 	"github.com/brimdata/super/vector/bitvec"
 )
 
-func castToType(sctx *super.Context, vec vector.Any, index []uint32) (vector.Any, []uint32, bool) {
+func castToType(sctx *super.Context, vec vector.Any, index []uint32) (vector.Any, []uint32, string, bool) {
 	switch vec := vec.(type) {
 	case *vector.TypeValue:
-		return vec, nil, true
+		return vec, nil, "", true
 	case *vector.String:
 		n := lengthOf(vec, index)
 		out := vector.NewTypeValueEmpty(0, bitvec.Zero)
@@ -36,8 +36,8 @@ func castToType(sctx *super.Context, vec vector.Any, index []uint32) (vector.Any
 			}
 			out.Append(val.Bytes())
 		}
-		return out, errs, true
+		return out, errs, "", true
 	default:
-		return nil, nil, false
+		return nil, nil, "", false
 	}
 }
diff --git a/runtime/ztests/expr/cast/string.yaml b/runtime/ztests/expr/cast/string.yaml
index 6877c71ed3..5d9bb90cb1 100644
--- a/runtime/ztests/expr/cast/string.yaml
+++ b/runtime/ztests/expr/cast/string.yaml
@@ -21,6 +21,8 @@ input: |
   1.2.3.4/32
   0x68692c20776f726c64
   0x666f6f20626172
+  0x00
+  0xc328
   "hi, world"
   "foo bar"
   {foo:"bar"}
@@ -46,6 +48,8 @@ output: |
   "1.2.3.4/32"
   "hi, world"
   "foo bar"
+  "\u0000"
+  error({message:"cannot cast to string: invalid UTF-8",on:0xc328})
   "hi, world"
   "foo bar"
   "{foo:\"bar\"}"