diff --git a/testing/protopack/pack.go b/testing/protopack/pack.go
index 5d28450..d39593a 100644
--- a/testing/protopack/pack.go
+++ b/testing/protopack/pack.go
@@ -586,7 +586,7 @@
 				case math.Float64bits(float64(math.NaN())) == math.Float64bits(float64(f)):
 					s = fmt.Sprintf("%T(math.NaN())", v)
 				default:
-					s = fmt.Sprintf("%T(math.Float64frombits(0x%08x))", v, math.Float64bits(float64(f)))
+					s = fmt.Sprintf("%T(math.Float64frombits(0x%016x))", v, math.Float64bits(float64(f)))
 				}
 				break
 			}