diff --git a/_generated/convert.go b/_generated/convert.go
index 964cff7d..9f0d92bb 100644
--- a/_generated/convert.go
+++ b/_generated/convert.go
@@ -81,3 +81,26 @@ type ConvertErrVal string
 type ConvertErr struct {
 	Err ConvertErrVal
 }
+
+//msgp:shim ConvertIntVal as:int64 using:fromConvertIntVal/toConvertIntVal mode:convert
+//msgp:ignore ConvertIntVal
+
+func fromConvertIntVal(v ConvertIntVal) (int64, error) {
+	return int64(v), nil
+}
+
+func toConvertIntVal(i int64) (ConvertIntVal, error) {
+	return ConvertIntVal(i), nil
+}
+
+type ConvertIntVal int64
+
+// ConvertInt exercises a fixed-size (int64) convert shim.
+type ConvertInt struct {
+	Int  ConvertIntVal
+	Ptr  *ConvertIntVal
+	Map  map[string]ConvertIntVal
+	MapP map[string]*ConvertIntVal
+	Arr  []ConvertIntVal
+	ArrP []*ConvertIntVal
+}
diff --git a/_generated/convert_test.go b/_generated/convert_test.go
index 7b67305e..6440f745 100644
--- a/_generated/convert_test.go
+++ b/_generated/convert_test.go
@@ -2,6 +2,7 @@ package _generated
 
 import (
 	"bytes"
+	"reflect"
 	"testing"
 
 	"github.com/tinylib/msgp/msgp"
@@ -58,3 +59,30 @@ func TestConvertToMarshalError(t *testing.T) {
 		t.Fatalf("expected conversion error, found %v", err.Error())
 	}
 }
+
+func TestConvertInt(t *testing.T) {
+	// A fixed-size convert shim must report an accurate constant Msgsize.
+	v := ConvertIntVal(7)
+	in := ConvertInt{
+		Int:  42,
+		Ptr:  &v,
+		Map:  map[string]ConvertIntVal{"a": 1},
+		MapP: map[string]*ConvertIntVal{"b": &v},
+		Arr:  []ConvertIntVal{1, 2},
+		ArrP: []*ConvertIntVal{&v},
+	}
+	b, err := in.MarshalMsg(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if in.Msgsize() < len(b) {
+		t.Fatalf("Msgsize %d under-reports marshaled size %d", in.Msgsize(), len(b))
+	}
+	var out ConvertInt
+	if _, err = out.UnmarshalMsg(b); err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(out, in) {
+		t.Fatalf("round-trip mismatch: %v != %v", out, in)
+	}
+}
diff --git a/gen/size.go b/gen/size.go
index 3efab37d..5b9639ae 100644
--- a/gen/size.go
+++ b/gen/size.go
@@ -228,6 +228,12 @@ func (s *sizeGen) gBase(b *BaseElem) {
 		return
 	}
 	if b.Convert && b.ShimMode == Convert {
+		if fixedSize(b.Value) {
+			// A fixed-size base has a constant wire size, so there is no need
+			// for a temporary holding the converted value.
+			s.addConstant(basesizeExpr(b.Value, "", b.BaseName()))
+			return
+		}
 		s.state = add
 		vname := randIdent()
 		s.p.printf("\nvar %s %s", vname, b.BaseType())
@@ -237,7 +243,6 @@ func (s *sizeGen) gBase(b *BaseElem) {
 
 		s.p.printf("\ns += %s", basesizeExpr(b.Value, vname, b.BaseName()))
 		s.state = expr
-
 	} else {
 		vname := b.Varname()
 		if b.Convert {