diff --git a/assembly.py b/assembly.py
index f1ed1b8..2d7e460 100644
--- a/assembly.py
+++ b/assembly.py
@@ -908,18 +908,11 @@ class AsmConstraint(AsmGroup):
         if len(sels)>1:
             raise RuntimeError(
                     'The selections must have a common (grand)parent assembly')
-        subs = sels[0].SubElementNames
-        if not subs:
-            raise RuntimeError('no sub-object in selection')
-        if len(subs)>2:
-            raise RuntimeError('too many selection')
-        if len(subs)==2:
-            sobj = sels[0].Object.getSubObject(subs[1],1)
-            if isTypeOf(sobj,Assembly,True) or \
-               isTypeOf(sobj,(AsmConstraintGroup,AsmConstraint)):
-                subs = (subs[1],subs[0])
-
         sel = sels[0]
+        subs = sel.SubElementNames
+        if not subs:
+            subs = ['']
+
         cstr = None
         elements = []
         elementInfo = []
@@ -931,22 +924,11 @@ class AsmConstraint(AsmGroup):
                 raise RuntimeError('Cannot find sub-object {}.{}'.format(
                     sel.Object.Name,sub))
             ret = Assembly.find(sel.Object,sub,
-                    recursive=True,relativeToChild=False)
+                    recursive=True,relativeToChild=False,keepEmptyChild=True)
             if not ret:
                 raise RuntimeError('Selection {}.{} is not from an '
                     'assembly'.format(sel.Object.Name,sub))
 
-            # check if the selection is a constraint group or a constraint
-            if isTypeOf(sobj,Assembly,True) or \
-               isTypeOf(sobj,(AsmConstraintGroup,Assembly,AsmConstraint)):
-                if assembly:
-                    raise RuntimeError('no element selection')
-                assembly = ret[-1].Assembly
-                selSubname = sub[:-len(ret[-1].Subname)]
-                if isTypeOf(sobj,AsmConstraint):
-                    cstr = sobj
-                continue
-
             if not assembly:
                 assembly = ret[0].Assembly
                 selSubname = sub[:-len(ret[0].Subname)]
@@ -962,14 +944,27 @@ class AsmConstraint(AsmGroup):
                         'assembly {}'.format(
                             sel.Object.Name,sub,objName(assembly)))
 
+            if isTypeOf(sobj,Assembly,True):
+                continue
+
+            # check if the selection is a constraint group or a constraint
+            if isTypeOf(sobj,Assembly,True) or \
+               isTypeOf(sobj,(AsmConstraintGroup,AsmConstraint)):
+                if isTypeOf(sobj,AsmConstraint):
+                    if cstr:
+                        raise RuntimeError('more than one constraint selected')
+                    assembly = ret[-1].Assembly
+                    selSubname = sub[:-len(ret[-1].Subname)]
+                    cstr = sobj
+                continue
+
+
             # because we call Assembly.find() above with relativeToChild=False,
             # we shall adjust the element subname by popping the first '.'
             sub = found.Subname
             sub = sub[sub.index('.')+1:]
             if sub[-1] == '.' and \
-               not isTypeOf(sobj,Assembly,True) and \
-               not isTypeOf(sobj,(AsmConstraint,AsmConstraintGroup,
-                                  AsmElement,AsmElementLink)):
+               not isTypeOf(sobj,(AsmElement,AsmElementLink)):
                 # Too bad, its a full selection, let's guess the sub element
                 if not utils.isElement((found.Object,sub)):
                     raise RuntimeError('no sub element (face, edge, vertex) in '
@@ -1680,6 +1675,8 @@ class AsmWorkPlane(object):
     def getSelection(sels=None):
         if not sels:
             sels = FreeCADGui.Selection.getSelectionEx('',False)
+        if not sels:
+            raise RuntimeError('no selection')
         if len(sels)!=1 or len(sels[0].SubElementNames)>1:
             raise RuntimeError('too many selections')
         if sels[0].SubElementNames:
diff --git a/constraint.py b/constraint.py
index ac1af6b..545894d 100644
--- a/constraint.py
+++ b/constraint.py
@@ -333,10 +333,10 @@ class ConstraintCommand:
         return self.tp._toolbarName
 
     def workbenchActivated(self):
-        pass
+        self._active = None
 
     def workbenchDeactivated(self):
-        self._active = None
+        pass
 
     def getContextMenuName(self):
         pass
@@ -739,8 +739,8 @@ class BaseMulti(Base):
     _entityDef = (_wa,)
 
     @classmethod
-    def check(cls,group,_checkCount=False):
-        if len(group)<2:
+    def check(cls,group,checkCount=False):
+        if checkCount and len(group)<2:
             raise RuntimeError('Constraint "{}" requires at least two '
                 'elements'.format(cls.getName()))
         for info in group:
@@ -1089,10 +1089,12 @@ class SketchPlane(BaseSketch):
 
     @classmethod
     def getEntityDef(cls,group,checkCount,obj=None):
+        _ = checkCount
+        _ = obj
         if not group:
             # If no element, then this constraint serves the prupose of clearing
             # the current sketch plane
-            return
+            return []
         # if there is any child element in this constraint, we expect the first
         # one to be a planar face or edge to define the work plane. The rest of
         # entities must be from some draft wire or circle/arc.