diff --git a/godeltaprof/compat/compression_test.go b/godeltaprof/compat/compression_test.go
index ceeb4b7..f2d3cda 100644
--- a/godeltaprof/compat/compression_test.go
+++ b/godeltaprof/compat/compression_test.go
@@ -10,9 +10,10 @@ func BenchmarkHeapCompression(b *testing.B) {
 	h := newHeapTestHelper()
 	fs := h.generateMemProfileRecords(512, 32)
 	b.ResetTimer()
+	nmutations := int(h.rng.Int63() % int64(len(fs)))
 	for i := 0; i < b.N; i++ {
 		_ = WriteHeapProto(h.dp, h.opt, io.Discard, fs, int64(runtime.MemProfileRate))
-		h.mutate(fs)
+		h.mutate(nmutations, fs)
 	}
 }
 
diff --git a/godeltaprof/compat/delta_test.go b/godeltaprof/compat/delta_test.go
index 6a122bf..4ae28ca 100644
--- a/godeltaprof/compat/delta_test.go
+++ b/godeltaprof/compat/delta_test.go
@@ -172,9 +172,10 @@ func BenchmarkHeapDelta(b *testing.B) {
 	fs := h.generateMemProfileRecords(512, 32)
 	builder := &noopBuilder{}
 	b.ResetTimer()
+	nmutations := int(h.rng.Int63() % int64(len(fs)))
 	for i := 0; i < b.N; i++ {
 		_ = h.dp.WriteHeapProto(builder, fs, int64(runtime.MemProfileRate))
-		h.mutate(fs)
+		h.mutate(nmutations, fs)
 	}
 }
 
diff --git a/godeltaprof/compat/testdata.go b/godeltaprof/compat/testdata.go
index fc9b082..5127db0 100644
--- a/godeltaprof/compat/testdata.go
+++ b/godeltaprof/compat/testdata.go
@@ -295,10 +295,9 @@ func (h *heapTestHelper) r(AllocObjects, AllocBytes, FreeObjects, FreeBytes int6
 	}
 }
 
-func (h *heapTestHelper) mutate(fs []runtime.MemProfileRecord) {
-	nmutations := int(h.rng.Int63() % int64(len(fs)))
+func (h *heapTestHelper) mutate(n int, fs []runtime.MemProfileRecord) {
 	objSize := fs[0].AllocBytes / fs[0].AllocObjects
-	for j := 0; j < nmutations; j++ {
+	for j := 0; j < n; j++ {
 		idx := int(uint(h.rng.Int63())) % len(fs)
 		fs[idx].AllocObjects += 1
 		fs[idx].AllocBytes += objSize