diff --git a/rpc/export.go b/rpc/export.go
index e8f00873..6119880b 100644
--- a/rpc/export.go
+++ b/rpc/export.go
@@ -72,7 +72,7 @@ func (c *lockedConn) releaseExport(id exportID, count uint32) (capnp.ClientSnaps
 		defer ent.cancel()
 		snapshot := ent.snapshot
 		c.lk.exports[id] = nil
-		c.lk.exportID.remove(uint32(id))
+		c.lk.exportID.remove(id)
 		metadata := snapshot.Metadata()
 		syncutil.With(metadata, func() {
 			c.clearExportID(metadata)
@@ -170,7 +170,7 @@ func (c *lockedConn) sendCap(d rpccp.CapDescriptor, snapshot capnp.ClientSnapsho
 			wireRefs: 1,
 			cancel:   func() {},
 		}
-		id = exportID(c.lk.exportID.next())
+		id = c.lk.exportID.next()
 		if int64(id) == int64(len(c.lk.exports)) {
 			c.lk.exports = append(c.lk.exports, ee)
 		} else {
@@ -308,7 +308,7 @@ func (e embargo) String() string {
 //
 // The caller must be holding onto c.mu.
 func (c *lockedConn) embargo(client capnp.Client) (embargoID, capnp.Client) {
-	id := embargoID(c.lk.embargoID.next())
+	id := c.lk.embargoID.next()
 	e := newEmbargo(client)
 	if int64(id) == int64(len(c.lk.embargoes)) {
 		c.lk.embargoes = append(c.lk.embargoes, e)
diff --git a/rpc/idgen.go b/rpc/idgen.go
index 56ff0bab..acdc72e4 100644
--- a/rpc/idgen.go
+++ b/rpc/idgen.go
@@ -3,15 +3,15 @@ package rpc
 // idgen returns a sequence of monotonically increasing IDs with
 // support for replacement.  The zero value is a generator that
 // starts at zero.
-type idgen struct {
+type idgen[T ~uint32] struct {
 	i    uint32
 	free uintSet
 }
 
-func (gen *idgen) next() uint32 {
+func (gen *idgen[T]) next() T {
 	if first, ok := gen.free.min(); ok {
 		gen.free.remove(first)
-		return uint32(first)
+		return T(first)
 	}
 	i := gen.i
 	if i == ^uint32(0) {
@@ -22,10 +22,10 @@ func (gen *idgen) next() uint32 {
 		panic("overflow ID")
 	}
 	gen.i++
-	return i
+	return T(i)
 }
 
-func (gen *idgen) remove(i uint32) {
+func (gen *idgen[T]) remove(i T) {
 	gen.free.add(uint(i))
 }
 
diff --git a/rpc/idgen_test.go b/rpc/idgen_test.go
index 3b710b36..8bd83e37 100644
--- a/rpc/idgen_test.go
+++ b/rpc/idgen_test.go
@@ -7,7 +7,7 @@ import (
 
 func TestIDGen(t *testing.T) {
 	t.Run("NoReplacement", func(t *testing.T) {
-		var gen idgen
+		var gen idgen[uint32]
 		for i := uint32(0); i <= 128; i++ {
 			got := gen.next()
 			if got != i {
@@ -16,7 +16,7 @@ func TestIDGen(t *testing.T) {
 		}
 	})
 	t.Run("Replacement", func(t *testing.T) {
-		var gen idgen
+		var gen idgen[uint32]
 		for i := 0; i < 64; i++ {
 			gen.next()
 		}
diff --git a/rpc/import.go b/rpc/import.go
index 65f327a9..f098e008 100644
--- a/rpc/import.go
+++ b/rpc/import.go
@@ -111,7 +111,7 @@ func (ic *importClient) Send(ctx context.Context, s capnp.Send) (*capnp.Answer,
 				})
 				q.p.Reject(rpcerr.WrapFailed("send message", err))
 				syncutil.With(&ic.c.lk, func() {
-					ic.c.lk.questionID.remove(uint32(q.id))
+					ic.c.lk.questionID.remove(q.id)
 				})
 				return
 			}
diff --git a/rpc/question.go b/rpc/question.go
index 458ba229..46953dfb 100644
--- a/rpc/question.go
+++ b/rpc/question.go
@@ -51,7 +51,7 @@ func (flags questionFlags) Contains(flag questionFlags) bool {
 func (c *lockedConn) newQuestion(method capnp.Method) *question {
 	q := &question{
 		c:             (*Conn)(c),
-		id:            questionID(c.lk.questionID.next()),
+		id:            c.lk.questionID.next(),
 		release:       func() {},
 		finishMsgSend: make(chan struct{}),
 	}
@@ -156,7 +156,7 @@ func (q *question) PipelineSend(ctx context.Context, transform []capnp.PipelineO
 				})
 				q2.p.Reject(rpcerr.WrapFailed("send message", err))
 				syncutil.With(&q.c.lk, func() {
-					q.c.lk.questionID.remove(uint32(q2.id))
+					q.c.lk.questionID.remove(q2.id)
 				})
 				return
 			}
diff --git a/rpc/rpc.go b/rpc/rpc.go
index 3d1e618e..d8e4b62e 100644
--- a/rpc/rpc.go
+++ b/rpc/rpc.go
@@ -122,13 +122,13 @@ type Conn struct {
 
 		// Tables
 		questions  []*question
-		questionID idgen
+		questionID idgen[questionID]
 		answers    map[answerID]*ansent
 		exports    []*expent
-		exportID   idgen
+		exportID   idgen[exportID]
 		imports    map[importID]*impent
 		embargoes  []*embargo
-		embargoID  idgen
+		embargoID  idgen[embargoID]
 	}
 }
 
@@ -330,7 +330,7 @@ func (c *Conn) Bootstrap(ctx context.Context) (bc capnp.Client) {
 				})
 				q.p.Reject(exc.Annotate("rpc", "bootstrap", err))
 				syncutil.With(&c.lk, func() {
-					c.lk.questionID.remove(uint32(q.id))
+					c.lk.questionID.remove(q.id)
 				})
 				return
 			}
@@ -1112,7 +1112,7 @@ func (c *Conn) handleReturn(ctx context.Context, in transport.IncomingMessage) e
 			select {
 			case <-q.finishMsgSend:
 				if q.flags.Contains(finishSent) {
-					c.lk.questionID.remove(uint32(qid))
+					c.lk.questionID.remove(qid)
 				}
 				dq.Defer(in.Release)
 			default:
@@ -1123,7 +1123,7 @@ func (c *Conn) handleReturn(ctx context.Context, in transport.IncomingMessage) e
 					<-q.finishMsgSend
 					c.withLocked(func(c *lockedConn) {
 						if q.flags.Contains(finishSent) {
-							c.lk.questionID.remove(uint32(qid))
+							c.lk.questionID.remove(qid)
 						}
 					})
 				}()
@@ -1183,7 +1183,7 @@ func (c *Conn) handleReturn(ctx context.Context, in transport.IncomingMessage) e
 						c.er.ReportError(err)
 					} else {
 						q.flags |= finishSent
-						c.lk.questionID.remove(uint32(qid))
+						c.lk.questionID.remove(qid)
 					}
 				})
 			})
@@ -1574,7 +1574,7 @@ func (c *Conn) handleDisembargo(ctx context.Context, in transport.IncomingMessag
 			if e != nil {
 				// TODO(soon): verify target matches the right import.
 				c.lk.embargoes[id] = nil
-				c.lk.embargoID.remove(uint32(id))
+				c.lk.embargoID.remove(id)
 			}
 		})
 		if e == nil {