diff --git a/zenmap/zenmapCore/NetworkInventory.py b/zenmap/zenmapCore/NetworkInventory.py
index dc28662..4fc0be6 100644
--- a/zenmap/zenmapCore/NetworkInventory.py
+++ b/zenmap/zenmapCore/NetworkInventory.py
@@ -445,7 +445,11 @@ class FilteredNetworkInventory(NetworkInventory):
         """A helper function that calls the matching function for the given
         operator and each of its arguments."""
         for arg in args:
-            if not self.__getattribute__("match_%s" % operator)(host, arg):
+            positive = True
+            if arg != "" and arg[0] == "!":
+                arg = arg[1:]
+                positive = False
+            if positive != self.__getattribute__("match_%s" % operator)(host, arg):
                 # No match for this operator
                 return False
         else:
diff --git a/zenmap/zenmapCore/SearchResult.py b/zenmap/zenmapCore/SearchResult.py
index 75c023f..640de29 100644
--- a/zenmap/zenmapCore/SearchResult.py
+++ b/zenmap/zenmapCore/SearchResult.py
@@ -200,7 +200,11 @@ class SearchResult(object):
         """A helper function that calls the matching function for the given
         operator and each of its arguments."""
         for arg in args:
-            if not self.__getattribute__("match_%s" % operator)(arg):
+            positive = True
+            if arg != "" and arg[0] == "!":
+                arg = arg[1:]
+                positive = False
+            if positive != self.__getattribute__("match_%s" % operator)(arg):
                 # No match for this operator
                 return False
         else: