diff --git a/internal/impl/codec_extension.go b/internal/impl/codec_extension.go
index 4bb0a7a2..0d5b546e 100644
--- a/internal/impl/codec_extension.go
+++ b/internal/impl/codec_extension.go
@@ -67,7 +67,6 @@ type lazyExtensionValue struct {
 	xi         *extensionFieldInfo
 	value      protoreflect.Value
 	b          []byte
-	fn         func() protoreflect.Value
 }
 
 type ExtensionField struct {
@@ -158,10 +157,9 @@ func (f *ExtensionField) lazyInit() {
 		}
 		f.lazy.value = val
 	} else {
-		f.lazy.value = f.lazy.fn()
+		panic("No support for lazy fns for ExtensionField")
 	}
 	f.lazy.xi = nil
-	f.lazy.fn = nil
 	f.lazy.b = nil
 	atomic.StoreUint32(&f.lazy.atomicOnce, 1)
 }
@@ -174,13 +172,6 @@ func (f *ExtensionField) Set(t protoreflect.ExtensionType, v protoreflect.Value)
 	f.lazy = nil
 }
 
-// SetLazy sets the type and a value that is to be lazily evaluated upon first use.
-// This must not be called concurrently.
-func (f *ExtensionField) SetLazy(t protoreflect.ExtensionType, fn func() protoreflect.Value) {
-	f.typ = t
-	f.lazy = &lazyExtensionValue{fn: fn}
-}
-
 // Value returns the value of the extension field.
 // This may be called concurrently.
 func (f *ExtensionField) Value() protoreflect.Value {