diff --git a/sphinx/util/docfields.py b/sphinx/util/docfields.py
index b59d3f317..f7b19c47d 100644
--- a/sphinx/util/docfields.py
+++ b/sphinx/util/docfields.py
@@ -219,7 +219,9 @@ class DocFieldTransformer(object):
             if typedesc is None or typedesc.has_arg != bool(fieldarg):
                 # either the field name is unknown, or the argument doesn't
                 # match the spec; capitalize field name and be done with it
-                new_fieldname = fieldtype.capitalize() + ' ' + fieldarg
+                new_fieldname = fieldtype[0:1].upper() + fieldtype[1:]
+                if fieldarg:
+                     new_fieldname = new_fieldname + ' ' + fieldarg
                 fieldname[0] = nodes.Text(new_fieldname)
                 entries.append(field)
                 continue
diff --git a/tests/root/objects.txt b/tests/root/objects.txt
index 533bdae3e..57e822127 100644
--- a/tests/root/objects.txt
+++ b/tests/root/objects.txt
@@ -7,6 +7,32 @@ Testing object descriptions
 
 .. function:: func_without_body()
 
+.. function:: func_with_unknown_field()
+
+   : :
+
+   : empty field name:
+
+   :field_name:
+
+   :field_name all lower:
+
+   :FIELD_NAME:
+
+   :FIELD_NAME ALL CAPS:
+
+   :Field_Name:
+
+   :Field_Name All Word Caps:
+
+   :Field_name:
+
+   :Field_name First word cap:
+
+   :FIELd_name:
+
+   :FIELd_name PARTial caps:
+
 .. function:: func_noindex
    :noindex:
 
diff --git a/tests/test_build_html.py b/tests/test_build_html.py
index 3813ed04d..aef733057 100644
--- a/tests/test_build_html.py
+++ b/tests/test_build_html.py
@@ -196,6 +196,17 @@ HTML_XPATH = {
             'Testing object descriptions'),
         (".//li[@class='toctree-l1']/a[@href='markup.html']",
             'Testing various markup'),
+        # test unknown field names
+        (".//th[@class='field-name']", 'Field_name:'),
+        (".//th[@class='field-name']", 'Field_name all lower:'),
+        (".//th[@class='field-name']", 'FIELD_NAME:'),
+        (".//th[@class='field-name']", 'FIELD_NAME ALL CAPS:'),
+        (".//th[@class='field-name']", 'Field_Name:'),
+        (".//th[@class='field-name']", 'Field_Name All Word Caps:'),
+        (".//th[@class='field-name']", 'Field_name:'),
+        (".//th[@class='field-name']", 'Field_name First word cap:'),
+        (".//th[@class='field-name']", 'FIELd_name:'),
+        (".//th[@class='field-name']", 'FIELd_name PARTial caps:'),
         # custom sidebar
         (".//h4", 'Custom sidebar'),
         # docfields