diff --git a/src/pynwb/data/nwb.base.yaml b/src/pynwb/data/nwb.base.yaml
index 2f4e62e8c..14ed915c5 100644
--- a/src/pynwb/data/nwb.base.yaml
+++ b/src/pynwb/data/nwb.base.yaml
@@ -38,7 +38,7 @@ datasets:
     value: indexes into a list of values for a list of elements
   default_name: vector_index
   shape:
-  -
+  - null
 - neurodata_type_def: ElementIdentifiers
   neurodata_type_inc: NWBData
   dtype: int
@@ -50,7 +50,7 @@ datasets:
     value: unique identifiers for a list of elements
   default_name: element_id
   shape:
-  -
+  - null
 groups:
 - neurodata_type_def: NWBContainer
   doc: The attributes specified here are included in all interfaces.
@@ -98,7 +98,7 @@ groups:
     - num_times
     quantity: '?'
     shape:
-    -
+    - null
   - name: control_description
     dtype: text
     doc: 'Description of each control value. COMMENT: Array length should be as long
@@ -108,7 +108,7 @@ groups:
     - num_control_values
     quantity: '?'
     shape:
-    -
+    - null
   - name: data
     doc: 'Data values. Can also store binary data (eg, image frames) COMMENT: This
       field may be a link to data stored in an external file, especially in the case
@@ -137,7 +137,7 @@ groups:
     dims:
     - num_times
     shape:
-    -
+    - null
   - name: starting_time
     dtype: float64
     doc: 'The timestamp of the first sample. COMMENT: When timestamps are uniformly
@@ -170,7 +170,7 @@ groups:
     - num_times
     quantity: '?'
     shape:
-    -
+    - null
   groups:
   - name: sync
     doc: 'Lab specific time and sync information as provided directly from hardware
@@ -245,7 +245,7 @@ groups:
     doc: The names of the columns in this table. This should be used to specifying
       an order to the columns
     shape:
-    -
+    - null
   - name: description
     dtype: text
     doc: Description of what is in this dynamic table
@@ -255,7 +255,7 @@ groups:
     dtype: int
     doc: The unique identifier for the rows in this dynamic table
     shape:
-    -
+    - null
   - neurodata_type_inc: TableColumn
     doc: The columns in this dynamic table
     quantity: '*'
diff --git a/src/pynwb/data/nwb.behavior.yaml b/src/pynwb/data/nwb.behavior.yaml
index 5417445b0..f92a62693 100644
--- a/src/pynwb/data/nwb.behavior.yaml
+++ b/src/pynwb/data/nwb.behavior.yaml
@@ -24,18 +24,18 @@ groups:
     attributes:
     - name: unit
       dtype: text
-      doc: 'The base unit of measure used to store data. This should be in the SI
-        unit. COMMENT: This is the SI unit (when appropriate) of the stored data,
-        such as Volts. If the actual data is stored in millivolts, the field ''conversion''
-        below describes how to convert the data to the specified SI unit.'
+      doc: "The base unit of measure used to store data. This should be in the SI\
+        \ unit. COMMENT: This is the SI unit (when appropriate) of the stored data,\
+        \ such as Volts. If the actual data is stored in millivolts, the field 'conversion'\
+        \ below describes how to convert the data to the specified SI unit."
       default_value: meter
       required: false
     dims:
     - num_times
     - num_features
     shape:
-    -
-    -
+    - null
+    - null
   - name: reference_frame
     dtype: text
     doc: Description defining what exactly 'straight-ahead' means.
diff --git a/src/pynwb/data/nwb.ecephys.yaml b/src/pynwb/data/nwb.ecephys.yaml
index a95fb22e4..f70b5d04f 100644
--- a/src/pynwb/data/nwb.ecephys.yaml
+++ b/src/pynwb/data/nwb.ecephys.yaml
@@ -84,9 +84,9 @@ groups:
     - - num_times
       - num_channels
     shape:
-    - -
-    - -
-      -
+    - - null
+    - - null
+      - null
   - neurodata_type_inc: ElectrodeTableRegion
     name: electrodes
     doc: the electrodes that this series was generated from
@@ -124,11 +124,11 @@ groups:
       - num_channels
       - num_samples
     shape:
-    - -
-      -
-    - -
-      -
-      -
+    - - null
+      - null
+    - - null
+      - null
+      - null
 - neurodata_type_def: ClusterWaveforms
   neurodata_type_inc: NWBDataInterface
   doc: 'The mean waveform shape, including standard deviation, of the different clusters.
@@ -157,8 +157,8 @@ groups:
     - num_clusters
     - num_samples
     shape:
-    -
-    -
+    - null
+    - null
   - name: waveform_sd
     dtype: float32
     doc: Stdev of waveforms for each cluster, using the same indices as in mean
@@ -166,8 +166,8 @@ groups:
     - num_clusters
     - num_samples
     shape:
-    -
-    -
+    - null
+    - null
   links:
   - name: clustering_interface
     doc: HDF5 link to Clustering interface that was the source of the clustered data
@@ -195,7 +195,7 @@ groups:
     dims:
     - num_events
     shape:
-    -
+    - null
   - name: peak_over_rms
     dtype: float32
     doc: Maximum ratio of waveform peak to RMS on any channel in the cluster (provides
@@ -203,7 +203,7 @@ groups:
     dims:
     - num_clusters
     shape:
-    -
+    - null
   - name: times
     dtype: float64
     doc: Times of clustered events, in seconds. This may be a link to times field
@@ -211,7 +211,7 @@ groups:
     dims:
     - num_events
     shape:
-    -
+    - null
   default_name: Clustering
 - neurodata_type_def: FeatureExtraction
   neurodata_type_inc: NWBDataInterface
@@ -229,7 +229,7 @@ groups:
     dims:
     - num_features
     shape:
-    -
+    - null
   - name: features
     dtype: float32
     doc: Multi-dimensional array of features extracted from each event.
@@ -238,16 +238,16 @@ groups:
     - num_channels
     - num_features
     shape:
-    -
-    -
-    -
+    - null
+    - null
+    - null
   - name: times
     dtype: float64
     doc: Times of events that features correspond to (can be a link).
     dims:
     - num_events
     shape:
-    -
+    - null
   - neurodata_type_inc: ElectrodeTableRegion
     name: electrodes
     doc: the electrodes that this series was generated from
@@ -274,7 +274,7 @@ groups:
     dims:
     - num_events
     shape:
-    -
+    - null
   - name: times
     dtype: float64
     doc: Timestamps of events, in Seconds
@@ -287,7 +287,7 @@ groups:
     dims:
     - num_events
     shape:
-    -
+    - null
   links:
   - name: source_electricalseries
     doc: HDF5 link to ElectricalSeries that this data was calculated from. Metadata
diff --git a/src/pynwb/data/nwb.file.yaml b/src/pynwb/data/nwb.file.yaml
index 4d3fc5bf1..a1b14003d 100644
--- a/src/pynwb/data/nwb.file.yaml
+++ b/src/pynwb/data/nwb.file.yaml
@@ -24,7 +24,7 @@ groups:
     dims:
     - '*unlimited*'
     shape:
-    -
+    - null
   - name: identifier
     dtype: text
     doc: 'A unique text identifier for the file. COMMENT: Eg, concatenated lab name,
@@ -255,7 +255,7 @@ groups:
           dims:
           - num_namespaces
           shape:
-          -
+          - null
         quantity: '*'
       quantity: '?'
     - neurodata_type_def: Subject
diff --git a/src/pynwb/data/nwb.icephys.yaml b/src/pynwb/data/nwb.icephys.yaml
index 6764791d4..1ed06d247 100644
--- a/src/pynwb/data/nwb.icephys.yaml
+++ b/src/pynwb/data/nwb.icephys.yaml
@@ -18,7 +18,7 @@ groups:
     dims:
     - num_times
     shape:
-    -
+    - null
   - name: gain
     dtype: float
     doc: 'Units: Volt/Amp (v-clamp) or Volt/Volt (c-clamp)'
diff --git a/src/pynwb/data/nwb.image.yaml b/src/pynwb/data/nwb.image.yaml
index 964b12bfe..cb563caa8 100644
--- a/src/pynwb/data/nwb.image.yaml
+++ b/src/pynwb/data/nwb.image.yaml
@@ -31,15 +31,15 @@ groups:
       - y
       - x
     shape:
-    - - 
-      - 
-    - - 
-      - 
-      - 
-    - - 
-      - 
-      - 
-      - 
+    - - null
+      - null
+    - - null
+      - null
+      - null
+    - - null
+      - null
+      - null
+      - null
   - name: dimension
     dtype: int32
     doc: Number of pixels on x, y, (and z) axes.
@@ -47,7 +47,7 @@ groups:
     - rank
     quantity: '?'
     shape:
-    - 
+    - null
   - name: external_file
     dtype: text
     doc: 'Path or URL to one or more external file(s). Field only present if format=external.
@@ -64,12 +64,12 @@ groups:
       dims:
       - num_files
       shape:
-      - 
+      - null
     dims:
     - num_files
     quantity: '?'
     shape:
-    - 
+    - null
   - name: format
     dtype: text
     doc: Format of image. If this is 'external' then the field external_file contains
@@ -149,7 +149,7 @@ groups:
     dims:
     - num_times
     shape:
-    - 
+    - null
   links:
   - name: indexed_timeseries
     doc: HDF5 link to TimeSeries containing images that are indexed.
diff --git a/src/pynwb/data/nwb.misc.yaml b/src/pynwb/data/nwb.misc.yaml
index 57d077f79..4decca683 100644
--- a/src/pynwb/data/nwb.misc.yaml
+++ b/src/pynwb/data/nwb.misc.yaml
@@ -33,8 +33,8 @@ groups:
     - num_times
     - num_features
     shape:
-    -
-    -
+    - null
+    - null
   - name: feature_units
     dtype: text
     doc: Units of each feature.
@@ -42,14 +42,14 @@ groups:
     - num_features
     quantity: '?'
     shape:
-    -
+    - null
   - name: features
     dtype: text
     doc: Description of the features represented in TimeSeries::data.
     dims:
     - num_features
     shape:
-    -
+    - null
 - neurodata_type_def: AnnotationSeries
   neurodata_type_inc: TimeSeries
   doc: Stores, eg, user annotations made during an experiment. The TimeSeries::data[]
@@ -77,7 +77,7 @@ groups:
     dims:
     - num_times
     shape:
-    -
+    - null
 - neurodata_type_def: IntervalSeries
   neurodata_type_inc: TimeSeries
   doc: Stores intervals of data. The timestamps field stores the beginning and end
@@ -108,7 +108,7 @@ groups:
     dims:
     - num_times
     shape:
-    -
+    - null
 - neurodata_type_def: UnitTimes
   neurodata_type_inc: NWBDataInterface
   doc: Event times of observed units (e.g. cell, synapse, etc.). The UnitTimes group
diff --git a/src/pynwb/data/nwb.ogen.yaml b/src/pynwb/data/nwb.ogen.yaml
index 4726188e4..9c54c2f85 100644
--- a/src/pynwb/data/nwb.ogen.yaml
+++ b/src/pynwb/data/nwb.ogen.yaml
@@ -23,7 +23,7 @@ groups:
     dims:
     - num_times
     shape:
-    -
+    - null
   links:
   - name: site
     doc: link to OptogeneticStimulusSite group that describes the site to which this
diff --git a/src/pynwb/data/nwb.ophys.yaml b/src/pynwb/data/nwb.ophys.yaml
index 5593c84a3..17002f326 100644
--- a/src/pynwb/data/nwb.ophys.yaml
+++ b/src/pynwb/data/nwb.ophys.yaml
@@ -13,9 +13,9 @@ datasets:
   - num_x
   - num_y
   shape:
-  -
-  -
-  -
+  - null
+  - null
+  - null
 - neurodata_type_def: PixelMasks
   neurodata_type_inc: VectorData
   dtype:
@@ -121,8 +121,8 @@ groups:
     - num_times
     - num_ROIs
     shape:
-    -
-    -
+    - null
+    - null
   - neurodata_type_inc: ROITableRegion
     name: rois
     doc: a dataset referencing into an ROITable containing information on the ROIs
@@ -263,8 +263,8 @@ groups:
     - weight
     - x|y|z
     shape:
-    -
-    -
+    - null
+    - null
     - 3
   - name: reference_frame
     dtype: text
diff --git a/src/pynwb/data/nwb.retinotopy.yaml b/src/pynwb/data/nwb.retinotopy.yaml
index a5cce3ab8..48fcf03b2 100644
--- a/src/pynwb/data/nwb.retinotopy.yaml
+++ b/src/pynwb/data/nwb.retinotopy.yaml
@@ -25,14 +25,14 @@ groups:
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: field_of_view
       dtype: float
       doc: Size of viewing area, in meters
       dims:
       - row|column
       shape:
-      - 
+      - null
     - name: unit
       dtype: text
       doc: Unit that axis data is stored in (e.g., degrees)
@@ -40,8 +40,8 @@ groups:
     - num_rows
     - num_cols
     shape:
-    - 
-    - 
+    - null
+    - null
   - name: axis_1_power_map
     dtype: float32
     doc: Power response on the first measured axis. Response is scaled so 0.0 is no
@@ -54,14 +54,14 @@ groups:
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: field_of_view
       dtype: float
       doc: Size of viewing area, in meters
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: unit
       dtype: text
       doc: Unit that axis data is stored in (e.g., degrees)
@@ -70,8 +70,8 @@ groups:
     - num_cols
     quantity: '?'
     shape:
-    - 
-    - 
+    - null
+    - null
   - name: axis_2_phase_map
     dtype: float32
     doc: Phase response to stimulus on the second measured axis
@@ -83,14 +83,14 @@ groups:
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: field_of_view
       dtype: float
       doc: Size of viewing area, in meters
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: unit
       dtype: text
       doc: Unit that axis data is stored in (e.g., degrees)
@@ -98,8 +98,8 @@ groups:
     - num_rows
     - num_cols
     shape:
-    - 
-    - 
+    - null
+    - null
   - name: axis_2_power_map
     dtype: float32
     doc: Power response on the second measured axis. Response is scaled so 0.0 is
@@ -112,14 +112,14 @@ groups:
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: field_of_view
       dtype: float
       doc: Size of viewing area, in meters
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: unit
       dtype: text
       doc: Unit that axis data is stored in (e.g., degrees)
@@ -128,8 +128,8 @@ groups:
     - num_cols
     quantity: '?'
     shape:
-    - 
-    - 
+    - null
+    - null
   - name: axis_descriptions
     dtype: text
     doc: Two-element array describing the contents of the two response axis fields.
@@ -138,7 +138,7 @@ groups:
     dims:
     - '2'
     shape:
-    - 
+    - null
   - name: focal_depth_image
     dtype: uint16
     doc: 'Gray-scale image taken with same settings/parameters (e.g., focal depth,
@@ -155,14 +155,14 @@ groups:
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: field_of_view
       dtype: float
       doc: Size of viewing area, in meters
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: focal_depth
       dtype: float
       doc: Focal depth offset, in meters
@@ -173,8 +173,8 @@ groups:
     - num_rows
     - num_cols
     shape:
-    - 
-    - 
+    - null
+    - null
   - name: sign_map
     dtype: float32
     doc: Sine of the angle between the direction of the gradient in axis_1 and axis_2
@@ -186,20 +186,20 @@ groups:
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: field_of_view
       dtype: float
       doc: Size of viewing area, in meters.
       dims:
       - row_col
       shape:
-      - 
+      - null
     dims:
     - num_rows
     - num_cols
     shape:
-    - 
-    - 
+    - null
+    - null
   - name: vasculature_image
     dtype: uint16
     doc: 'Gray-scale anatomical image of cortical surface. Array structure: [rows][columns]'
@@ -215,14 +215,14 @@ groups:
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: field_of_view
       dtype: float
       doc: Size of viewing area, in meters
       dims:
       - row_col
       shape:
-      - 
+      - null
     - name: format
       dtype: text
       doc: Format of image. Right now only 'raw' supported
@@ -230,6 +230,6 @@ groups:
     - num_rows
     - num_cols
     shape:
-    - 
-    - 
+    - null
+    - null
   default_name: ImagingRetinotopy
diff --git a/src/pynwb/form/spec/write.py b/src/pynwb/form/spec/write.py
index 08eea0c90..dcb319058 100644
--- a/src/pynwb/form/spec/write.py
+++ b/src/pynwb/form/spec/write.py
@@ -52,6 +52,12 @@ def reorder_yaml(self, path):
             f_out.write(yaml.dump(sorted_data, Dumper=yaml.RoundTripDumper))
 
     def sort_keys(self, obj):
+
+        # Represent None as null
+        def my_represent_none(self, data):
+            return self.represent_scalar(u'tag:yaml.org,2002:null', u'null')
+        yaml.RoundTripRepresenter.add_representer(type(None), my_represent_none)
+
         order = ['neurodata_type_def', 'neurodata_type_inc', 'name', 'dtype', 'doc',
                  'attributes', 'datasets', 'groups']
         if isinstance(obj, dict):