diff --git a/api/layer/writer.go b/api/layer/writer.go
index 833a794e..2290e391 100644
--- a/api/layer/writer.go
+++ b/api/layer/writer.go
@@ -12,7 +12,7 @@ type offsetWriter struct {
 	length int64
 }
 
-func newWriter(w io.Writer, offset, length int64) io.Writer {
+func newWriter(w io.Writer, offset, length int64) *offsetWriter {
 	return &offsetWriter{
 		Writer: w,
 		offset: offset,
diff --git a/api/layer/writer_test.go b/api/layer/writer_test.go
index 1a242f64..762f1dd6 100644
--- a/api/layer/writer_test.go
+++ b/api/layer/writer_test.go
@@ -33,10 +33,8 @@ func TestOffsetWriter(t *testing.T) {
 			require.NoError(t, err)
 		}
 
-		wo := wt.(*offsetWriter)
-
-		require.Equal(t, o, wo.skipped)
-		require.Equal(t, l, wo.written)
+		require.Equal(t, o, wt.skipped)
+		require.Equal(t, l, wt.written)
 		require.Equal(t, b[o:o+l], w.Bytes())
 	})
 
@@ -51,10 +49,8 @@ func TestOffsetWriter(t *testing.T) {
 			require.NoError(t, err)
 		}
 
-		wo := wt.(*offsetWriter)
-
-		require.Equal(t, o, wo.skipped)
-		require.Equal(t, l, wo.written)
+		require.Equal(t, o, wt.skipped)
+		require.Equal(t, l, wt.written)
 		require.Equal(t, b[o:o+l], w.Bytes())
 	})
 
@@ -69,10 +65,8 @@ func TestOffsetWriter(t *testing.T) {
 			require.NoError(t, err)
 		}
 
-		wo := wt.(*offsetWriter)
-
-		require.Equal(t, o, wo.skipped)
-		require.Equal(t, l, wo.written)
+		require.Equal(t, o, wt.skipped)
+		require.Equal(t, l, wt.written)
 		require.Equal(t, b[o:o+l], w.Bytes())
 	})
 
@@ -87,10 +81,8 @@ func TestOffsetWriter(t *testing.T) {
 			require.NoError(t, err)
 		}
 
-		wo := wt.(*offsetWriter)
-
-		require.Equal(t, o, wo.skipped)
-		require.Equal(t, s, wo.written)
+		require.Equal(t, o, wt.skipped)
+		require.Equal(t, s, wt.written)
 		require.Equal(t, b, w.Bytes())
 	})
 
@@ -105,10 +97,8 @@ func TestOffsetWriter(t *testing.T) {
 			require.NoError(t, err)
 		}
 
-		wo := wt.(*offsetWriter)
-
-		require.Equal(t, o, wo.skipped)
-		require.Equal(t, s, wo.written)
+		require.Equal(t, o, wt.skipped)
+		require.Equal(t, s, wt.written)
 		require.Equal(t, b, w.Bytes())
 	})
 }