diff --git a/src/z3c/rml/canvas.py b/src/z3c/rml/canvas.py
index 6b2a78a..c41b761 100644
--- a/src/z3c/rml/canvas.py
+++ b/src/z3c/rml/canvas.py
@@ -343,7 +343,8 @@ class IImage(interfaces.IRMLDirectiveSignature):
 
     mask = attr.Color(
         title=u'Mask',
-        description=u'The color mask used to render the image.',
+        description=u'The color mask used to render the image, or "auto" to use the alpha channel if available.',
+        default='auto',
         required=False,
         acceptAuto=True)
 
diff --git a/src/z3c/rml/flowable.py b/src/z3c/rml/flowable.py
index f524880..e7ed956 100644
--- a/src/z3c/rml/flowable.py
+++ b/src/z3c/rml/flowable.py
@@ -903,7 +903,8 @@ class IImage(interfaces.IRMLDirectiveSignature):
 
     mask = attr.Color(
         title=u'Mask',
-        description=u'The color mask used to render the image.',
+        description=u'The color mask used to render the image, or "auto" to use the alpha channel if available.',
+        default='auto',
         required=False,
         acceptAuto=True)
 
@@ -980,7 +981,8 @@ class IImageAndFlowables(interfaces.IRMLDirectiveSignature):
 
     imageMask = attr.Color(
         title=u'Mask',
-        description=u'The height the image.',
+        description=u'The color mask used to render the image, or "auto" to use the alpha channel if available.',
+        default='auto',
         required=False,
         acceptAuto=True)
 
diff --git a/src/z3c/rml/tests/expected/tag-image-mask.pdf b/src/z3c/rml/tests/expected/tag-image-mask.pdf
index d58726b..fea1360 100644
Binary files a/src/z3c/rml/tests/expected/tag-image-mask.pdf and b/src/z3c/rml/tests/expected/tag-image-mask.pdf differ
diff --git a/src/z3c/rml/tests/input/tag-image-mask.rml b/src/z3c/rml/tests/input/tag-image-mask.rml
index 4b2d42a..f91bfb2 100644
--- a/src/z3c/rml/tests/input/tag-image-mask.rml
+++ b/src/z3c/rml/tests/input/tag-image-mask.rml
@@ -1,13 +1,20 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <!DOCTYPE document SYSTEM "rml.dtd">
 
-<document
-    filename="tag-image-svg.pdf"
-    xmlns:doc="http://namespaces.zope.org/rml/doc">
-  <pageDrawing>
+<document filename="tag-image-mask.pdf">
 
-    <image file="[z3c.rml.tests]/input/images/cylinder.png"
-        x="10mm" y="10mm" mask="auto" />
+  <template>
+    <pageTemplate id="main">
+        <pageGraphics>
+            <image file="[z3c.rml.tests]/input/images/cylinder.png" x="10mm" y="10mm" mask="auto" />
+            <image file="[z3c.rml.tests]/input/images/cylinder.png" x="30mm" y="10mm" />
+        </pageGraphics>
+      <frame id="first" x1="72" y1="72" width="451" height="698"/>
+    </pageTemplate>
+  </template>
 
-  </pageDrawing>
+  <story>
+    <img src="[z3c.rml.tests]/input/images/cylinder.png" width="20mm" height="20mm" mask="auto" />
+    <img src="[z3c.rml.tests]/input/images/cylinder.png" width="20mm" height="20mm" />
+  </story>
 </document>
diff --git a/src/z3c/rml/tests/test_rml.py b/src/z3c/rml/tests/test_rml.py
index ba9c92d..8809630 100644
--- a/src/z3c/rml/tests/test_rml.py
+++ b/src/z3c/rml/tests/test_rml.py
@@ -134,9 +134,6 @@ def test_suite():
        if not filename.endswith(".rml"):
            continue
 
-       if 'image-mask' not in filename:
-           continue
-
        inPath = os.path.join(inputDir, filename)
        outPath = os.path.join(outputDir, filename[:-4] + '.pdf')
        expectPath = os.path.join(expectDir, filename[:-4] + '.pdf')