diff --git a/rules/proxy/proxy_empty_ua.yml b/rules/proxy/proxy_empty_ua.yml
index faac0f93b..da28473a1 100644
--- a/rules/proxy/proxy_empty_ua.yml
+++ b/rules/proxy/proxy_empty_ua.yml
@@ -8,9 +8,8 @@ logsource:
     category: proxy
 detection:
     selection:
-      UserAgent:
-        # Empty string - as used by Powershell's (New-Object Net.WebClient).DownloadString 
-        - ''
+      # Empty string - as used by Powershell's (New-Object Net.WebClient).DownloadString 
+      UserAgent: ''
     condition: selection
 fields:
     - ClientIP
diff --git a/tools/sigma/backends/elasticsearch.py b/tools/sigma/backends/elasticsearch.py
index c15d5a3ce..dbb51d105 100644
--- a/tools/sigma/backends/elasticsearch.py
+++ b/tools/sigma/backends/elasticsearch.py
@@ -26,7 +26,7 @@ class ElasticsearchQuerystringBackend(SingleTextQueryBackend):
     identifier = "es-qs"
     active = True
 
-    reEscape = re.compile("([+\\-=!(){}\\[\\]^\"~:/]|\\\\(?![*?])|\\\\u|&&|\\|\\|)")
+    reEscape = re.compile("([\\s+\\-=!(){}\\[\\]^\"~:/]|\\\\(?![*?])|\\\\u|&&|\\|\\|)")
     reClear = re.compile("[<>]")
     andToken = " AND "
     orToken = " OR "
@@ -34,12 +34,19 @@ class ElasticsearchQuerystringBackend(SingleTextQueryBackend):
     subExpression = "(%s)"
     listExpression = "(%s)"
     listSeparator = " "
-    valueExpression = "\"%s\""
+    valueExpression = "%s"
     nullExpression = "NOT _exists_:%s"
     notNullExpression = "_exists_:%s"
     mapExpression = "%s:%s"
     mapListsSpecialHandling = False
 
+    def generateValueNode(self, node):
+        result = super().generateValueNode(node)
+        if result == "" or result.isspace():
+            return '""'
+        else:
+            return result
+
 class ElasticsearchDSLBackend(RulenameCommentMixin, BaseBackend):
     """ElasticSearch DSL backend"""
     identifier = 'es-dsl'
diff --git a/tools/sigma/parser/condition.py b/tools/sigma/parser/condition.py
index 16e4d5675..ef697e90b 100644
--- a/tools/sigma/parser/condition.py
+++ b/tools/sigma/parser/condition.py
@@ -24,6 +24,24 @@ COND_OR   = 2
 COND_NOT  = 3
 COND_NULL = 4
 
+# Debugging code
+def dumpNode(node, indent=''):   # pragma: no cover
+    """
+    Recursively print the AST rooted at *node* for debugging.
+    """
+    if hasattr(node, 'items'):
+        print("%s%s<%s>" % (indent, type(node).__name__,
+                            type(node.items).__name__))
+        if type(node.items) != list:
+            dumpNode(node.items, indent + '  ')
+        else:
+            for item in node.items:
+                dumpNode(item, indent + '  ')
+    else:
+        print("%s%s=%s" % (indent, type(node).__name__,
+                                   repr(node)))
+    return node
+
 # Condition Tokenizer
 class SigmaConditionToken:
     """Token of a Sigma condition expression"""
@@ -271,23 +289,6 @@ class SigmaConditionOptimizer:
     """
     Optimizer for the parsed AST.
     """
-    def _dumpNode(self, node, indent=''):   # pragma: no cover
-        """
-        Recursively print the AST rooted at *node* for debugging.
-        """
-        if hasattr(node, 'items'):
-            print("%s%s<%s>" % (indent, type(node).__name__,
-                                type(node.items).__name__))
-            if type(node.items) != list:
-                self._dumpNode(node.items, indent + '  ')
-            else:
-                for item in node.items:
-                    self._dumpNode(item, indent + '  ')
-        else:
-            print("%s%s=%s" % (indent, type(node).__name__,
-                                       repr(node)))
-        return node
-
     def _stripSubexpressionNode(self, node):
         """
         Recursively strips all subexpressions (i.e. brackets) from the AST.
diff --git a/tools/sigma/parser/rule.py b/tools/sigma/parser/rule.py
index 6560aaa33..565562405 100644
--- a/tools/sigma/parser/rule.py
+++ b/tools/sigma/parser/rule.py
@@ -87,12 +87,6 @@ class SigmaParser:
                         fields = [ fields ]
                     for field in fields:
                         cond.add(ConditionNULLValue(val=field))
-                elif value == "not null":
-                    fields = mapping.resolve_fieldname(key)
-                    if type(fields) == str:
-                        fields = [ fields ]
-                    for field in fields:
-                        cond.add(ConditionNotNULLValue(val=field))
                 else:
                     cond.add(mapping.resolve(key, value, self))