diff --git a/rasterio/tools/merge.py b/rasterio/tools/merge.py
index 858ddb39..07d9c411 100644
--- a/rasterio/tools/merge.py
+++ b/rasterio/tools/merge.py
@@ -8,7 +8,7 @@ from rasterio._base import get_index, get_window
 from rasterio.transform import Affine
 
 
-def merge(sources, bounds=(), res=(), nodata=None):
+def merge(sources, bounds=None, res=None, nodata=None):
     """Copy valid pixels from input files to an output file.
 
     All files must have the same number of bands, data type, and
diff --git a/rasterio/transform.py b/rasterio/transform.py
index c02bc1cc..b8327116 100644
--- a/rasterio/transform.py
+++ b/rasterio/transform.py
@@ -2,6 +2,7 @@ import warnings
 
 from affine import Affine
 
+
 IDENTITY = Affine.identity()
 
 
@@ -38,3 +39,11 @@ def from_bounds(west, south, east, north, width, height):
     `height` in number of pixels."""
     return Affine.translation(west, north) * Affine.scale(
             (east - west)/width, (south - north)/height)
+
+
+def array_bounds(height, width, transform):
+    """Return the `west, south, east, north` bounds of an array given
+    its height, width, and an affine transform."""
+    w, n = transform.xoff, transform.yoff
+    e, s = transform * (width, height)
+    return w, s, e, n
diff --git a/tests/test_rio_merge.py b/tests/test_rio_merge.py
index d492373c..95f7bc6f 100644
--- a/tests/test_rio_merge.py
+++ b/tests/test_rio_merge.py
@@ -95,6 +95,7 @@ def test_merge_warn(test_data_dir_1):
     runner = CliRunner()
     result = runner.invoke(merge, inputs + [outputname] + ['--nodata', '-1'])
     assert result.exit_code == 0
+    assert os.path.exists(outputname)
     assert "using the --nodata option for better results" in result.output