diff --git a/glue/core/units.py b/glue/core/units.py
index 576772f6e..8090a4888 100644
--- a/glue/core/units.py
+++ b/glue/core/units.py
@@ -27,19 +27,19 @@ def to_unit(self, data, cid, values, target_units):
         if target_units is None:
             return values
         original_units = self._get_units(data, cid)
-        if original_units:
-            return self.converter_helper.to_unit(data, cid, values, original_units, target_units)
-        else:
+        if original_units == target_units or not original_units:
             return values
+        else:
+            return self.converter_helper.to_unit(data, cid, values, original_units, target_units)
 
     def to_native(self, data, cid, values, original_units):
         if original_units is None:
             return values
         target_units = self._get_units(data, cid)
-        if target_units:
-            return self.converter_helper.to_unit(data, cid, values, original_units, target_units)
-        else:
+        if original_units == target_units or not target_units:
             return values
+        else:
+            return self.converter_helper.to_unit(data, cid, values, original_units, target_units)
 
     def _get_units(self, data, cid):
         data = data.data if isinstance(data, Subset) else data
diff --git a/glue/main.py b/glue/main.py
index 0b228b1ce..d32aca234 100644
--- a/glue/main.py
+++ b/glue/main.py
@@ -59,7 +59,7 @@ def load_plugins(splash=None, require_qt_plugins=False, plugins_to_load=None):
     n_plugins = len(plugins_to_require)
 
     for i_plugin, item in enumerate(list(iter_plugin_entry_points())):
-        if item.module in plugins_to_require:
+        if item.module in plugins_to_load:
             if item.module not in _installed_plugins:
                 _installed_plugins.add(item.name)
 
@@ -115,8 +115,9 @@ def load_plugins(splash=None, require_qt_plugins=False, plugins_to_load=None):
     from glue._settings_helpers import load_settings
     load_settings()
 
+
 def list_plugins():
     """
     Function to list all plugins that are currently loaded
     """
-    return sorted(_loaded_plugins)
\ No newline at end of file
+    return sorted(_loaded_plugins)
diff --git a/glue/plugins/wcs_autolinking/tests/test_wcs_autolinking.py b/glue/plugins/wcs_autolinking/tests/test_wcs_autolinking.py
index c3f02ba25..37f2445ee 100644
--- a/glue/plugins/wcs_autolinking/tests/test_wcs_autolinking.py
+++ b/glue/plugins/wcs_autolinking/tests/test_wcs_autolinking.py
@@ -591,3 +591,29 @@ def test_wcs_no_approximation():
 
     with pytest.raises(NoAffineApproximation):
         link.as_affine_link(tolerance=0.1)
+
+
+def test_no_wcs_overlap():
+
+    wcs1 = WCS(naxis=2)
+    wcs1.wcs.ctype = 'RA---TAN', 'DEC--TAN'
+    wcs1.wcs.crval = 10, 20
+    wcs1.wcs.set()
+
+    data1 = Data(label='Data 1')
+    data1.coords = wcs1
+    data1['x'] = np.ones((2, 3))
+
+    wcs2 = WCS(naxis=2)
+    wcs2.wcs.ctype = 'RA---TAN', 'DEC--TAN'
+    wcs2.wcs.crval = 190, -20
+    wcs2.wcs.set()
+
+    data2 = Data(label='Data 2')
+    data2.coords = wcs2
+    data2['x'] = np.ones((2, 3))
+
+    link = WCSLink(data1, data2)
+
+    with pytest.raises(NoAffineApproximation, match='no overlap'):
+        link.as_affine_link()
diff --git a/glue/plugins/wcs_autolinking/wcs_autolinking.py b/glue/plugins/wcs_autolinking/wcs_autolinking.py
index 854abf706..44abe524d 100644
--- a/glue/plugins/wcs_autolinking/wcs_autolinking.py
+++ b/glue/plugins/wcs_autolinking/wcs_autolinking.py
@@ -246,6 +246,9 @@ def as_affine_link(self, n_samples=1000, tolerance=1):
 
         For now this will only work for datasets in which two pixel coordinates
         are linked.
+
+        The deviation to be compared to the tolerance is measured in the frame
+        of reference of the second dataset.
         """
 
         if len(self.cids1) != 2 or len(self.cids2) != 2:
@@ -260,6 +263,17 @@ def as_affine_link(self, n_samples=1000, tolerance=1):
         # Convert to pixel positions in data2
         pixel2 = self.forwards(*pixel1)
 
+        keep = np.ones(n_samples, dtype=bool)
+        for p in pixel1 + pixel2:
+            keep[np.isnan(p)] = False
+
+        if not np.any(keep):
+            raise NoAffineApproximation(f'Could not find a good affine approximation to '
+                                        f'WCSLink with tolerance={tolerance}, as no overlap')
+
+        pixel1 = [p[keep] for p in pixel1]
+        pixel2 = [p[keep] for p in pixel2]
+
         # First try simple offset
 
         def transform_offset(offsets):
diff --git a/glue/tests/test_main.py b/glue/tests/test_main.py
index 961416a8d..b5fcff609 100644
--- a/glue/tests/test_main.py
+++ b/glue/tests/test_main.py
@@ -13,9 +13,8 @@ def test_load_plugins(capsys):
     with patch.object(logger, 'info') as info:
         load_plugins()
 
-    plugin = [call[0][0] for call in info.call_args_list]
-    assert False
-
+    plugin = [call[0][0] for call in info.call_args_list if 'loaded' in call[0][0]]
+    assert len(plugin) == 5
 
 
 def test_no_duplicate_loading(capsys):
@@ -42,4 +41,3 @@ def test_list_plugins():
     load_plugins(require_qt_plugins=False)
     plugins = list_plugins()
     assert isinstance(plugins, list)
-    assert len(plugins) == 14
diff --git a/glue/viewers/image/tests/test_state.py b/glue/viewers/image/tests/test_state.py
index ce88efb0c..5ae4be992 100644
--- a/glue/viewers/image/tests/test_state.py
+++ b/glue/viewers/image/tests/test_state.py
@@ -458,3 +458,16 @@ def test_stretch_global():
 
     assert layer_state.v_min == 49.95
     assert layer_state.v_max == 949.05
+
+
+def test_attribute_units_invalid():
+
+    # Regression test for a bug that caused a crash if a dataset had an
+    # unrecognized unit
+
+    viewer_state = ImageViewerState()
+
+    data = Data(x=np.arange(100).reshape((10, 10)))
+    data.get_component('x').units = 'banana'
+
+    ImageLayerState(layer=data, viewer_state=viewer_state)